I think well-roundedness of skills is unknown as far as training (the rules suggest that keeping skills in balance will help training in related skills).
Balance, especially between offense and defense skills might help win games, especially considering that only 5 players are on the court at a time.
I have an idea that salary might reflect marketable skills, which just as in RL don't precisely reflect playing value, and may not be the same as the market evaluates a player.
In 5 seasons, the 18 YO will be the no-brainer to be trained. The 23 YO would be someone who was drafted and sat on the end of the bench or in street clothes for 4 or 5 seasons, But since no one is trained might have potential. On the other hand, in 3 seasons would you want to buy a 26 YO or 21 YO trained player, even if the current 23 YO did maintain some of his skill advantage.