There should be a hard cap on how many minutes a player could stay on court depending on his age.
Players up to 27 years should be able to play as many minutes as they are requested, thus not affecting training.
However, older players shouldn't be allowed to play as much.
This would affect mostly those infamous teams who have short rosters made of monsters with huge salaries (usuallly on their prime, which in BB happen between 28-33, from when they finish their training to when they start decreasing skills).
If you can have 5-6 monsters who play 48 minutes per game without any major penalties (ok, stamina affects performance and stuff, but they can still play and produce to a considerable extent), why bother hiring backups or reserves?
However, if your 250k power forward can only play for 36 minutes, someone necessarily will need to play those 12 minutes left. Ok, you can put a 3k scrub to fill those minutes, but the performance will decrease a lot! So then managers will be left with a true need to fill their rosters with more balanced players to play the precious minutes left to the backups.
This would make finantially much harder to assemble and to keep a roster composed only of a few monsters and address more efficiently and sharply this matter.
Furthermore, IRL the player's ability to stay on court decreases with age, so this would be realistic.
36 year olds = max 17 min/game + 1 min/OT
35 year olds = max 21 min/game + 1 min/OT
34 year olds = max 25 min/game + 1 min/OT
33 year olds = max 29 min/game + 2 min/OT
32 year olds = max 33 min/game + 2 min/OT
31 year olds = max 36 min/game + 3 min/OT
30 year olds = max 39 min/game +3 min/OT
29 year olds = max 43 min/game + 4 min/OT
28 year olds = max 45 min/game + 4 min/OT
27 or less year olds = no time limit