Can we stop tossing names out there as though they prove a point? Larry Bird, Kevin Garnett, LeBron James, Freddy Adu. Maybe Freddy Adu's high school coach would have molded him to become the next Pele. Maybe his college coach would have shot him in the chest. If you're really interested in whether things would have been better or worse for that guy, you don't have any way of knowing. You can't point to a successful player and infer that he would have been more or less successful the other way around.
You have a 100-year-old uncle who smoked a pack a day. That doesn't mean smoking is a good system.
Jordan played three years of college and became the greatest that ever lived. That doesn't mean college is a good system.
The fact of the matter is, this is an argument about principle, not about quality of play. NBA GMs will make the most entertaining product they can with the pool of personnel available. The quality of that product can only go down when you restrict the pool.