Since the topic is still going on...
Improving Art above level 30 is extremely tedious, costly and/or risky. Timewise, it will take between 2 (best case scenario) to 4 seasons just to progress by one level, so it begs the question, is it worth it ?
Thus... Before discussing how prevalent are high level books and mage with 40+ in one Art, I like to look at the question "Why would a mage invest so much resources ?".
An Art in itself is only a tool to perform magic, thus, most mages develop an Art because they have a goal in mind that requires them to have a certain skill level, not for the sake of having the highest score (with a few exception for some Bonisagus).
So for those two to four seasons required to progress by one level, it would usually be more efficient to improve the second Art involved in the magical research, improve the laboratory, train leadership to have more than one assistant/apprentice, progress in a mystery cult to have a specific initiation and make sure that the familiar is as competent as possible.
The cost to study from vis become prohibitive at the same time as the risk for botch increases: once level 30 is reached, it costs 6 pawns, 7 dices to roll in case of 0 on the stress roll, just for an average progress of 10 to 12 points (considering an aura of 4 to 6). If the mage needs to repeat that three times to progress by one level, that's a small fortune.
For most case, once a mage reaches 20-25 he is golden for most of his research. There is only Hermetic Architecture Aura boosting that requires truly high level and for such endeavor, getting a magical focus in Aura manipulation would be worth a lot more than a couple extra levels. Look at Magi of Hermes to have example of mages designed with a goal or an interest in mind that is not just "achieving the highest art level possible".
That's one plausible coherent explanation that can be served to players.
Now, for the mechanical / meta-game explanation I found:
- The higher the quality of books, the higher the level of Summae, the more abundant Tractatus of good quality are, the less relevant young mages (= PCs) are: with such resources at disposal, the overall average power level of mages is significantly higher. Thus most young mages should focus on catching up with the middle of the pack if they want to have any meaningful impact, otherwise they will be out-Certamen, out-spelled, out-skilled by all but the youngest mages. It is still the case in the current setting, but the step between "average power" and young mage is lower, thus a mage does not need to dedicate so much time to become relevant.
It is like you just graduated with a master, but because everybody has at least one (if not more) PhD, if not several post-docs, nobody takes you seriously. - by design, the Order in the setting is flawed. It is a feature, not a bug. It gives so much opportunities for players to make a difference. How is it possible that the "guild" that gathers arguably the brightest minds of Europe, with resources that any kings would envy, plus powers that can only be matched by God if he decides to intervene is not better organised ? Why is there not a few academies where teaching of future apprentices is not handled by skilled teachers (Improved Theban Tribunal)? Why is there no better spread of knowledge in the Order (ie: collection of best tractati, copies of Primer Q21L6 in each Tribunal,...), etc. The metagame answer is simple: to leave something for the players to do!
While one can consider that overtime, better tractatus should be available on any topics, one should also consider how it would impact the whole background. And usually, it means it is a lot more difficult to stand out when the average level is higher. And as player, I want that my PC becomes relevant sooner than later, even if it is only in his small niche specialties. More powerful mages means less area unexplored, less original spells to invent, less opportunity to be original... Do you really want to say to your players that spend several seasons or years working on a nice project: "Decent job, but magus Emeritus did it fifty years ago with additional bells and whistles, you get a B-"...
So besides some very academic-oriented mages of the Bonisagus persuasion and a few other specific cases, my take is that although possible, the cost/benefit of getting truly high Art becomes unacceptable for most mages past a certain level.