True, however elite HS prospects ala a KD, OJ, Greg Oden, D Rose, John Wall, etc. (just naming guys off the top of my head who were highly touted coming out of hs) aren't improving their game much by going to play on the college level. Guys like that do not need college. It simply isn't necessary for them IMO. I don't see how KD would have gotten any better playing another year of college.
I think players who aren't highly touted or aren't NBA caliber out of High school, their games will for sure improve by staying in college more. But there are prospects like the aforementioned who are ready for the NBA right out of High School, and so it becomes a thing where you have to question how does college benefit those elite guys?
From an entertainment standpoint, yes the NBA has the right to say "we don't want you" but what people fail to understand is most rookies are going to struggle regardless of how long they stay in college. You aren't going to get players ready to contribute in a big way right away unless they are elite prospects like the aforementioned.