When you are a top 3 pick and possibly the #1 overall pick what do you have left to prove. Staying can only hurt him if he proves after another year that he still can't defend worth a lick.
Or if he gets injured, back injury, ACL tear, whatever. His stock can only go down really. Sully would have been top 5 if he had come out a year earlier. But dropped to 21 due to the back injury concerns. That's a lot of $$ lost.
Really up to him if he stays, but as far as basketball at the college level, he has nothing left to prove. Going back to finish his degree? He can always do that later. Such as going to summer school and evening classes.