On a pro level, what dictates what they can or cant do is the team offensive and defensive phylosophies, those are nowdays based on facts and facts are sustained with meassurable data ... meassurable data is of what ratings and tendencies should be based of ... you said that how do we know what the full potential of a payer is, I ask you how does 2k knows, how does +30 or +2 does the trick, why not +1 or +3 0r -5 ...
Again, as long as 2k sticks to its way of managing its roster and updates and subsequently its difficulty levels, we are bound for unrealistic gameplay, which is nothing other than unsim play ... we need a frequently automathed system, based on meassurable data, that can help us better portrait players/team oncourt behaviors ... we need certaint ratings to relate to certaint phylosophies, like higher defensive awareness making players double team hot opponents from the strong side, make them help on D on missmatches if their own man is out of his shooting range, that high on ball defense ratings may lean opponents onto help D, a higher offensive awareness makes players try and create missmatches, makes them more conscious of their strength and weaknesses, etc ...
What we are asking is, to base the distinction of levels on both, the reduced timing for actions (already applied) and the application of those phylosophies rather than ratings edits ... anyways, given that difficulty levels blur the signature nature of players and teams (realism), whatever approach is taken, is going to be a controversy, I understand that ... then again, since we already have sliders to alter ratings, why not take a shot at this approach, when many people are asking for a different take.