That it might be impossible to to invent a new LR for old magi due to the limif on vis/MT is not something that needs errata. Players have been aware of that for a good long while, and it is just one of those things one need to be aware of.
By the time a magus reaches an age where it starts to become difficult to find someone able to invent a new LR for them, they are almost certain to already have a LR that can be renewed, even if it might not be the best possible LR.
Hmm, so mundanes having a magus develop and re-perform a Longevity Ritual for them are stuck with the (Magic Theory x 2) of the magus even when the Ritual gets re-performed?
That does change a lot of dynamics, in particular in regard to Redcaps.
Don't think so. Other way around I suspect.
The previous phrasing could easily be misread as the Arts limit only applied when re-doing a LR for others. The clarification is presumably that the Arts limit applies not only when doing it for someone else, but for the magus too.
Ah, I see now. Ok, that makes sense 
Yes.
I have some thoughts on maximum MT scores. You say that a score of 16 should be extremely rare, no more than one or two in the history of the order. I guess that depends on how many older magi there are. The various tribunal books identify a number of magi of that age or greater and that's just one ones alive now. It doesn't count the ones from the last few centuries. I've seen estimates that say there are ~75 magi 150 years old or older in 1220. Let's use that as a ballpark number. If we assume a magus can make it to 150 years old, I have to wonder what he's done in that time. Assuming gauntlet at 25, that's 125 years or 500 seasons to play with.
As we all know, time is a magus's most valuable commodity, but then again, MT is one of the most useful things a magus can learn. I'm also assuming that with thousands of magi across the ages, there are a lot of MT tractatus out there, even counting the ones that have been lost or hidden. It has to be one of the most common tractatus ever written, and one of the most generally sought after. Every maga will want a tractatus for a Form or Technique that they prefer, but they all want MT!
Consider a 150 year old Bonisagus magus with Book Learner and Puissant MT, who starts with a 3+2 in MT at gauntlet. Let's assume he has been able to access a level 6 MT summa at level 8 and, over 125 years, one quality 13 MT tractatus, two quality 12 MT tractatus, three quality 11 MT tractatus, six quality 10 MT tractatus, ten quality 9 MT tractatus, and thirteen quality 8 MT tractatus. Yes, tractatus above quality 10 are rare, but six in 125 years (~ one every two decades) doesn't seem unreasonable, especially for a Bonisagus who may have access to Durenmar. Also, 35 MT tractatus over the course of more than a century does not seem too much. That's fewer than one every three years.
So, it's 9 seasons of study to get to level 6+2 MT with an L6/Q8 summa (which seems extremely reasonable to access) and 35 seasons of study to get to level 14+2 MT with the tractatus listed above, or a total of 44 seasons of study. Given the 150 year old magus had 500 seasons to work with by age 150, that's only 9% of his time. Basically, the magus has to study an MT tractatus about once every three years. That really doesn't seem unreasonable. In fact, it seems entirely likely for a Bonisagus magus.
Even if we go by the very conservative rules set forth in the core book (30 xp/year) a 14+2 MT is entirely possible. Getting to level 14+2 in MT by age 150 would require a magus to devote about 13% of his resources to MT. That's a significant percentage (and more than 9%), I'll admit, but not a crazy amount given how useful a high MT is. I don't argue that every magus of that age would have a super high MT, but given how many older magi there are, I have to believe that there would be a few each generation who went that route. Assuming only one or two in the order's entire history seems much, much too conservative, particularly given how valuable a high MT can be.
I know people tend to look at players gaming the system as being a power gaming thing that actual magi wouldn't do, but let's be real. Have you looked at how people in the real world game things like the financial system for their own personal gain? If insider trading weren't illegal it would be rampant. Don't you think that a maga who was really good at writing (Good Teacher and a high COM) would work with a clever Corpus magus (high Creo/Corpus with Book Learner) to scratch each other's backs? The writer could write maybe five or six quality 12 or 13 MT tractatus and several Cr or Co tractatus of a similar quality and offer to let the other magus read them in exchange for making a longevity ritual for her. (Bonus, she still has the tractatus at the end of the scheme to sell or lend out again.) Secrecy be damned, the good writer is talking about extending her life by decades! She's not going to throw that away just because she doesn't want to share her toys. And if that good writer knew just two other good writers she could recruit them into the scheme to get the number of excellent tractatus up to 15-18 and shave another five seasons off the prep time (as well as up the reading magus's Cr and Co scores). What wouldn't a maga do for an extra fifty years of life? Devoting 44 seasons to something to get 200 seasons back seems like a good investment. (And the high MT is not just to get a longevity ritual; it has many, many more benefits!)
People might also say that I'm assuming the studying magus has Book Learner. Well, that's true. But if I were trying to predict how many excellent hard scientists there were in a modern generation, we might find that most of them were good at math. It's just that people who are good at math are more likely to do better at the hard sciences as compared to those people who are not good at math. They're also more likely to pursue a career in the hard sciences. So too, those magi with Book Learner will be the ones who pursue advancement through reading books. By and large, a maga with Book Learner will almost certainly read no fewer books than someone without Book Learner. They certainly have every incentive to read books in preference to other ways of advancing their abilities and arts. As a result, the magi drawn to this scheme will certainly include those with Book Learner.
People may also say that I'm assuming not only that my magus has Book Learner but that he's in that small group of older magi. True, but let's consider that a magus with Book Learner might have a higher MT for the reasons set forth above. That magus will be able to manage a longevity ritual later in life because he can handle more vis in a season and so will likely have a stronger longevity ritual (because his arts will be higher), which will allow him to live longer, which will allow him to continue to raise his MT, which will allow him to invent an even stronger longevity ritual, which will allow him to live longer, which ... well, you get the idea.
So, I conclude that while I don't believe that there will ever be a lot of magi with an MT of 16 (or 14+2), I do believe that more than one or two magi in history will likely have done it. I could believe that there might only be a half a percent who achieve that lofty goal, but that's still half a dozen magi in the order each generation.
ETA: and I didn't even consider how much Affinity with MT would improver things. Granted, then I'm assuming Puissant, Affinity, and Book Learner, which may be a stretch. But given that Puissant MT is given as a house virtue, it's not quite so bad.
When I am saying that it should be very rare with a score of 16+ in MT, I am not basing that on theorycrafting or trying to figuring out what an ambitious player character can get up to.
I am basing it on the characters in the published books.
Barring the archangels, I don't think there is a single statted individual in any of the books that has a score of 16+ in any (non-Accelerated) Ability. Even scores of 12+ are very rare in the published materials.
I take your point. I guess my response is that looking at what real people do in the real world, I have to conclude that the characters in the published books are intentionally kept weaker. The writers have clearly made decisions that do not track with how real people would have acted were they actually put in similar situations.
There are very good reasons to do that--play balance being the obvious one. Magi may be intentionally kept at a lower power than they could easily be so that the older magi don't overshadow the younger magi that PCs typically play. So, I don't criticize what's being done. However, I do recognize that it's really an artificial nerfing of older magi.
It's kind of like if wealth were a major part of the game and characters were only ever made with an income between £20,000-£100,000 a year. We know for a fact that some people take actions that earn them millions of pounds a year (or more). There are some people in the real world who make more every year than the GNP of some countries. However, in a game it might be unbalancing to have a person who makes that much money. It could totally overwhelm other players and allow one character to solve every problem. The game might well, either officially or unofficially, limit wealth to a narrower range.
If that's the case with MT, then I totally understand. I guess the pig-headed part of me just wants people to admit that its an artificial restriction added to the game for game balance purposes. If there are no magi with super high MT values it's not because it would be very difficult for a magus to achieve such an ability score; it's because we don't want very many (any?) characters to have such a score.
I guess I'm trying to say that I believe it isn't the case that "normal" characters would almost never be able to achieve super high ability scores but you could probably do it if you gamed the system. Rather, I think it's a case that some (albeit a few, much older) characters could achieve such high scores with a modicum of effort. However, for valid game reasons we don't generally do that.
Although I agree that Magic Theory is quite useful in a variety of situation, having a high score in it is rather costly, time-wise, compared to other ways to improve your lab total for those specific projects a magus might be pursuing. The only area in which MT cannot be replaced is for the seasonal vis limit. And that is only a cap for enchantments, including longevity rituals and usually excluding one's talisman. Even Original Research doesn't require a high score in MT.
So for magi who are more interested in inventing spells, binding a familiar and improving their talisman, increasing their Arts is a more effective in improving the relevant lab total. That may be why most of the characters presented in the various books don't have super-high scores in MT -- they are simply more interested in things where having a high MT score isn't critical.
Very true. In most cases it's more efficient to raise your Arts. Although MT applies to everything, which gives it an advantage, most people do focus their magi on only a few Arts. And once you have a medium-high value for MT you can always raise an Art for fewer xp. Even if your main TeFo is high enough that it would be cheaper to raise MT, there are always other useful Arts that could be raised more cheaply.
However, let's also keep in mind that if you're trying to get insight to develop a breakthrough you need to spend a season investigating a source of insight and make a stress roll of Intelligence + Magic Theory against an Ease Factor of 18. That's a pretty high EF. And if you fail the roll, it's a lost season. No one likes lost seasons. Given that, if you're trying for a breakthrough, there's a real incentive to get your MT high enough that your chance of failure is below 50%. (I'd argue most people would want it below 25%.) Even assuming a 3 INT, that means you'd want your MT as high as you could get it to do that kind of research. Having a value of 10-15 would not be unreasonable.
So, I totally agree that most magi will not have any reason to raise their MT that high. Instead they'll focus on other things and be more well-rounded. However, there are a few kinds of magi that will want a high MT. And for those magi it wouldn't be that hard to get an MT that high, given enough years. And while "most" magi might include 99.5% of the magi in existence, that 0.5% of magi who might well want to pursue that option represents 6 living magi (and some number of magi who have either died or passed to Final Twilight). That's a lot more than "perhaps one or two magi throughout the history of the Order."
I'm not saying that a lot of magi would do it or that it would even be reasonable for most magi to do it. What I am saying is that it would be relatively straightforward for a magus to get a very high MT score if they had a reason to do so and the time to study, and I have to believe that over the course of centuries with thousands of magi trained, there would have been a score--maybe a couple of dozen--magi with a good reasons to get a super high MT score who would have actually done so. Not a lot, but enough that people would know it could be done and enough that there might reasonably be a few level 8 MT summa out there. Of course there's no guarantee that the quality of such a summa would be any good. You might be better off just tracking down tractatus at that level.
Well. There are a few things to consider.
a) We are talking here about the older 10% of magi in the Order; canonically, there are over 100 of them, and (though a numbe of considerations) we can estimate them to be 100+ years beyond their gauntlet. Yet there are very, very few published magi with full stats who are 100+ years after their gauntlet. So, published characters do not really offer a fair sample.
b) We know that an ability of 16 is the equivalent, xp-wise, of an Art of 36(18). This is considered sufficiently "common" that not only high quality Summae exist at the corresponding level, but when creating older characters, a magus 110 years after his gauntlet is not considered "unbalanced" with such a score, even in the absence of Affinities, Puissant Art etc.
c) In fact, we know that level 8 summae on abilities, of high quality, can be purchased with build points. This means that the "standard" setting assumption is that a fair number of characters have reached the score of 16 necessary to write them - enough that at least a few very good authors are among them.
One factor is this- the characters stated in books tend to people of narrative interest. people playing at hermetic politics, or with grand schemes outside of books and laboratories. like I said, scores above 11 or 12 would be rare for adventuring magi because you wind up creating spells you can't cast 9at least outside of a strong aura).
But for obsessive researchers this does not apply. Verditius and Bonisagus especially are going to be more interested in magic theory, but the real experts in magic theory are going to be the ones who are least interesting in terms of game play, generally speaking. Thre could be some narrative exceptions- the research wizard who wants a more adventurous wizard to find certain inspirations for them, for example, could initiate a quest, but again the research wizard doesn't really need to be stated out.
Not appearing in the game and not existing in it are two separate things. You don't see a lot of peasant farmers stated out in the game either, but they most definitely exist.
My experience as a grad student leads me to agree with Arthur. Even if a longevity ritual is keeping them young I figure most magi have a fairly limited time of 50 years on so in which they are really pushing the limits. After that its puttering around in areas outside your main interests, hanging out at the magical equivalent of conferences, and simply enjoying the fact that they have hit the jackpot in life and are now effectively tenured, emeritus profs with no need to publish or get funding.
The Gift doesn't select for interest in book learning and theory. I figure there are a fair number of magi who would sound like Edison; it's not a lost season because they now know something that doesn't work! We, as game players, don't like lost seasons but I'm not convinced that magi would be so worried about it.
Seems fairly reasonable.