First off, I'm a long-time lurker and first-time poster
Second, I'm a new OOTP 8 owner and have only gone through about 6 seasons, so take what I say with a grain of salt.
I've read this thread through about the 9th page, and am at work, so I replied without completing it, and apologize in advance if I repeat someone else's post.
Based on what I've read, the developer didn't state that stats (performance) are the ONLY thing that determines talent changes, but are one component of it. I would say that makes sense, because different people react differently to success and/or failure; by tinker with batting stances, or pitching motions, or increases/decreases in confidence, plate patience, etc.; and these reactions could change one's abilities and potential.
Also, I haven't seen anyone comment on this aspect of it, but keep in mind that in baseball, everything truly is relative. It's not like track performers who compete against a track and a stopwatch. These are people competing against each other, so while a person's individual abilities may remain close to the same, they may vary widely when compared to other people at one's position, given the player shuffling and development of others.
Now, if a guy approaching his peak was hitting .325 and on pace for a 50+ homer season, and overnight turns into a .215 hitter who won't hit but more than 4-5 homers a year, and is entirely random, that could be a problem if it is a common occurrence. This is where my "newness" comes into play, because I haven't seen that happen yet, although that doesn't mean it hasn't happened (and I missed it) or it won't happen in the future.
Anyway, this is a fascinating discussion to read, and I'm glad to add my 2 cents (sorry for the length).