Well the fact is that the vast majority of teams don't train well. New players, afk players and half interested players probably make up 2/3 of all players in this game, they fill the lower leagues and there are legions of them.
Then in the middle divisions there are lots of teams that are not interested in training, they don't like that aspect of the game, or are in a too tight league to risk it or they just think training GS each week gives their team such a massive advantage that they stick with that.
Then there are those that try but for whatever reason should be good but are just incompetent.
Finally I estimate that leaves about 10% who actually train properly.
Seeing as those 10% are looking for good prospects from the other 90% no wonder that you can pick up a $4.5k superstar with good skills right after the draft for $10k.
I think everyone is so different at training that even if you gave everyone EXACTLY the same guy they would turn out differently. But they wont be. Even if everyone got a $4k all star each season there will be different heights, different skill balances, different aggerssion wrecking some that constantly foul out.
Obviously this would flood the market with $4k allstars so their value would crash from $5 to $1 and a lot of them would be fired instead of the $2k 6th men but would that really upset the balance of the game?