I personally dont have a major problem with the training system, but I have suggested 1 minor thing, that is, a training camp (probably in the last week of the season) to train players.
Why? Well the "off-season" should be a time when players can improve their game faster than normal, you can see this in the NBA in some cases, someone puts in work during the off season and improves drastically, whether it's free throws, defense etc.
Perhaps at a cost (100k/150k/200k/250k etc per player/position/type of training or intensity etc - Perhaps more expensive = more probability of pops, from a range of 1 to 5 pops or so - 5 pops is a lot, yes i know)
For one thing, the free throw ability of many players is deplorable (apart from the elites), as is the stamina.
So someone can train this in camps maybe to give their player an advantage with high FTs/Stamina.
Could also train the other skills as well of course.
I think while slightly speeding up training (probably adds about 1-1.5 seasons of training on top of the already trained attributes if a 21 year old was in a training camp every season).
This would add a lot of strategy to training and also financial development, saving money for these camps.
benefits? Well U21 could become more competitive. Maybe a HOFer who had weaker starting skills than someone else may not get discarded and he could catch up.
I think a problem as well as that the gap between "graduating" from U21s until hitting the NT is way too long. Shouldnt take 7+ years? To become a NT player.
It would add a new dynamic to the game, younger elite players, better NTs, better players on a whole basis.
As for inflation, I think this would improve the number of high quality players around, if there's a lot of high quality players around, then less money will be thrown around since there is less competition for a large pool compared to a smaller pool of talent.
Obviously my suggestion may have to be tailored slightly, to ensure the system cannot be abused (i dont think it can, since this would require long term investment into trainees) but I do think my general idea could be effective.