Not to pick nits, but not every lab is in a sanctum, and someone selling Longevity Potions for a living has good incentive to maintain a laboratory outside his sanctum (and a heavily specialized one at that) for the very reason you cite.
Magi living 100 years past gauntlet is probably average once you account for the effects of violence and spell botches. Each decade adds 1 to the roll, but each magnitude of Longevity subtracts 1, so to 'break even' and merely apparently age isn't that hard. If you're not interested in vanity and willing to accept that your stats will degrade, even a 10-12 magnitudes of Longevity can support you quite well. Magi capable of generating a CrCo lab total of 60 (that's including your assistance, naturally) will not be hard to find.
The other point is that the model was not intended to describe an average magus, but one whose demand for Tractatus is at the upper end- the maximum not average situation. combine with that the fact that it would be extremely rare to find a library (personal or covenant) which stocks more than 20 Tractatus in a single topic, and my point becomes very clear- there will always be a demand for high quality Tractatus. Even getting 56 Tractatus at level 11 to get from level 20 to 40 will be fairly difficult.
I had a Mercere specialist generating mag 20 LR on an average roll. And he only specialized in CrCo spells. I could have added more for specializing in LR.
That point wasn't really clear. I think we are all aware that there is always going to be a demand for sound or high quality tractatus. One of the key features of a tractatus, though, is that once used it can be easily traded for a different tractatus that you haven't already read. So that can be a reason that there aren't many libraries with more than 20 tractatus on a single topic is probably because they don't need to be kept if they have been used by everyone. One of the things that Covenants tried to do, but didn't really flesh out very well is how much it costs to keep a library running, but since a lot of covenants are rich, and don't like to deal with the silver based accounting, it's basically ignored. My point here, is that one would think it would cost a lot to maintain a library with many volumes. Enough to start charging for access, like Triamore and Durenmar, and probably Valnastium.
Cr25 Co25 Magic Theory 12 and a Focus plus misc stuff produces a lab total around 100 easily. That's not even close to upper end, it's something a middle aged LR specialist can achieve. With the right set up (such as yours, presumably), there isn't even a need for the Focus. One of our earliest campaigns had a group of young magi work together to create LRs in the 80 range.
Renowned LR specialists probably push 120 before the buyer's assistance. Theoretical maximums are far higher, but they're just theoretical hyper-optimized setups that probably don't actually occur.
It's not as if the majority of magi aren't, for all intents and purposes, already retired. In the modern sense retirement is becoming that period of life where you can focus on what you would love to do, or at least get away from the soul-crushing work that has drug you down for lo these past 30 years. Magi, by and large, get to choose their work...
Worse. Real life for a great many. My mom worked a job she hated to pay the bills. I'm lucky to not be in the same place, but I'd love to have enough to live on and devote myself to different endeavors of my choosing.
Yeah, but there's an expectation to participate in the politics of the Order and do stuff for your House (unless you're a Merinitia or ex Miscellanea).
A magus "retiring" probably means he's going into an eremitage somewhere to live comfortably for the rest of his life without having to deal with Quaesitores, covenant service, fights over vis, or making the next big spell for Durenmar. Just like a nobleman or priest might retire to a monastery or a peasant might turn his farm over to his sons to sit by the fire in a rocking chair.
Or a Mystery Cult. Barns Bjornaer, serve your clan! Criamon, answer riddles! Verditius, make stuff and fight vendettas!
Or a Societas. Flambeau, fight! Jerbiton, make art! Tytalus, survive your parens' and peers' constant harassment!
You're right that it's the True Lineages and Flambeau that are expected to serve the Order as a whole, but the others still have social obligations to their House.
It's not, actually. Eremites are a thing in the Order; magi who want to just live apart from other magi have existed since the dawn of the Order. Redcaps will still probably drop in from time to time out of courtesy, and they'll notify the Quaesitores if something really skeevy is going on or if the magus' actions are endangering other magi. Otherwise, it's no different from a Redcap settling down to run an inn.
I think it depends on what you want. Do you want book trading to work as a economic activity? Or do you want book trading to work as a story activity?
Regardless of what you choose, the rules will serve you poorly, with different books implying different economies.
What do I mean by economic activity? I mean straightforward transactions, exchanging some combination of goods, money and (maybe) services for a desired resource. For this to work, we need to assess the value of books relative to each other and to other goods.
What do I mean by story activity? I mean interactions where the exchange of value is not significant compared to other factors, such as a good relationship, a threat, completing a mission or quest, plundering a library, mutual assistance against an adversary, etc. For this to work, we need stories in which the acquisition of texts makes sense.
I'll not discuss the story category any further, other than to note that this works best in games where players are not very interested in economic activity at all. Want a trade? Fine, an NPC covenant will trade with you, and if the trade is unequal in some respect, one side "Owes the other," and there will be a story to make everything good. Or the NPC covenant will trade texts, but for some McGuffin you don't yet have.
Hmm. Maybe I will discuss this a bit further: I proposed some years ago abstracting libraries to "N xps per year," and have noticed various groups successfully working with that for character development. We can take this further, including trade into this abstraction. Rather than a "Name of the Rose" focus on "We have a guarded library with these and those rare texts," an "Internet Age" focus on interaction and communication among cultivators and accumulators of knowledge. (I am not suggesting anachronistic behavior, but a more up to date understanding of what actually went on 13c CE.) Then, covenant virtues and flaws can affect the number of xps per year, and the kind of extra stories that can interrupt a covenant's connection to the rest of the Order, and the larger conversation that is the development of Hermetic Magic. Perhaps some personal virtues and flaws too. (Book Learner has its usual effect; as usual, beware of stacking.)
Trading texts then becomes a background activity, except for story events that force a particular trade into the foreground as a story. You don't just need a text, you need The Book of Gibberish. No one wants to talk to your covenant while it is under investigation for diabolism. A pesky Crusade is interfering with the mail. A sudden plague has killed half the Redcaps in the Tribunal; even after you (or others) have figured out what happened and dealt with it as best you can, things will run less smoothly for a while. For the past few years, an archmagus at Durenmar has been corresponding with various members of your covenant, for reasons you don't yet understand, resulting in +1 SQ for study; is it worth trying to figure out his ulterior motive and risk alienating him? Etc.
But I suspect you are interested in an economic solution, and that's not trivial. Vis has different value in different Tribunals. Worse, different authors of AM books made different economic assumptions, and if there was some canon, this has drifted over the years. Thus, in one book we have House Bonisagus release works described as the best of the best that seemed like total crap, and that are total crap based on the rules in Covenants. Also worse, AM describes a few different economies (vis, Labor Points, silver, abstract categories from Poor to Legendary, maybe others) that really don't work well together. (Economy is difficult in an rpg, and few care enough about this to actually make sense, especially when most other rules are more important. AM is far from unique in this regard; D&D economy is vastly silly.)
If you want to simply assume that trading books is straightforward, and are willing to simplify the available Source Qualities, you can do something like the following:
There are 20 different tractati of SQ 10 available for each Art and each standard Arcane Ability for Hermetic Magic. These are valued at 1 pawn/text.
There are 5 different tractati of SQ 13 available as above, valued at 2pawn/text.
There are any number of vain tractati of SQ6 available as above. They are valued at 1pawn/text, even though they suck. Presumably, you aren't going to bother reading these unless you are desperate, and desperate people pay more. (Besides, it is well known that it is a Bad Idea to insult Archmagus Xwlerkmlgywerwer the Incomprehensible or his books. This never ends well.)
If you want summae, create a few specific summae at whatever levels you want, at SQ8, SQ12 and SQ16. The value of each, respectively, is 1/2/4 pawns per season of study the text provides at the full SQ, assuming starting at score 0 and no virtues or flaws that affect book learning.
Books for spell mastery are not generally available. You might make an exception for Pilum of Fire, and a few other spells that are known to have a long-standing and fanatical following.
Lab texts for spells or items might be valued at, say: 1 pawn for 20 levels of effect, either 1 effect or singly. A single effect of level 21-40 is 2 pawns, 41-50 is 3 pawns, and +1 pawn per 1-5 levels above 50. But it might also be reasonable to assume that spells in the core rules are so common, that you can get 50 levels for a pawn, mix and match.
Non-magical books in general circulation are available at 1 pounds of silver each, per rules in Covenants (or A&A)? A&A has a list of well-known titles; the values ascribed to these seem quite silly to me, but there's something to be said for canon. In Hibernia, maybe these cost 2 pounds if you want to enforce the idea that Hibernia is a backwater in 1220.