Hasheem Thabeet spent 3 years in college. He must be averaging 20 and 10 in the NBA, right? How about Michael Olowokandi? Kwame Brown is a bust, but he still played in the league for a decade.
The fact is players miss from high school and they miss from college. Players are ready from high school and they're ready from college. The NBA draft is always a crap shoot and GMs will always make bad picks because *shockingly* not every player in the draft will be a star. The age limit doesn't change any of that and it doesn't improve the end product in any significant way.
By the way, I realize you never saw Bird or Magic play, but you do understand the NBA is a very, very different game today. The rules have changed, the players have changed, the game has changed. Hell, in the late-90s teams averaged almost 15 points less a game than in the mid-80s. To compare raw numbers and try to draw anything about "readiness" is foolish. Not to mention you cherry picked 3 Hall of Famers that had extraordinary rookie seasons. Scottie Pippen averaged 7 points a game as a rookie. Kevin McHale 10 ppg, The Chief 9 ppg. I suppose these guys should have tried to play 5 years in college because they were clearly not ready for the NBA and the product was suffering.
I realize not every player who stays in college for three years is going to be a stud, just as not every kid who leaves early is going to be a bust. The problem is that you're comparing players with broad spectrums of talent. Comparing KG to Hasheem Thabeet and using that to support your argument isn't really valid, because one of those players is obviously 10X more talented than the other.
I'll conceed to you the fact that LeBron James, Dwight Howard, etc. would NOT have been better served attending college. But what I'll ask is this:
If Ray Allen (to use just one example) had gone to the NBA after one season, would he be the same sure fire HOFer he is today? Of course, there is no way to know for sure, but I strongly doubt it.
Conversley, if LeBron James had attended college for two years, would he be an worse off than he is today? Possibly he might have missed out on a few million, but he likely would have entered the league more mature, and with a better head on his shoulders when a difficult decision arises (no pun intended). I'm certainly not here to argue that LeBron needed college, but I don't think it could have hurt as much as it would have hurt a guy like Allen or Duncan if he had lept straight to the NBA.
My point is this: You need one rule that extends throughout the spectrum so that your product is as flawless as possible. LeBron James and Kobe Bryant were going to be great if they went to college or not. The age limit wouldn't be raised to help guys like them, it would be raised to help the future Tim Duncan's and Ray Allen's of the world. The guys with exceptional talent who need a place to harness and develop it before taking the next step.
For every LeBron James, there is a misguided teenager who is bound to make the jump due to a variety of factors, the biggsest likely being a shortage of money and people without his best interests in mind pulling him in one direction. The NBA swallows those kids and spits them back out with lost confidence and a greatly reduced chance that they'll ever amount to anything in the NBA. By raising the age limit, you eliminate the chance of a team giving away minutes to an umprepared rookie, and you increase the chances that they will one day meet their potential as a contributor to the NBA.
So in conclusion, would you rather miss out on LeBron's rookie and sophmore years in the NBA, or a spectacular career from a Dwyane Wade, Ray Allen, or Tim Duncan? Obviously we'll never know for sure how those guys would have translated to the NBA had they not spent multiple years in college, but I have a strong feeling they wouldn't have been the same players. You may say, "Well, nobody is forcing those guys to go directly to the NBA". That's true, but outside voices and temptations often force players into a decision they'll later regret. When you were in high school did you move on to algebra 3 before you had figured out algebra 2? While certain players can contribute in the NBA as rookies, there is clearly value in not skipping steps.
And I didn't "cherry pick" three Hall of Famers at all. I compared three of the best players of one generation (Magic, Bird, Jordan) to four of the best players of another generation (Howard, Garnett, Kobe, LeBron). I think it's fair to see how their rookie seasons stack up against one another, considering three went to college for multpile years and four didn't go to college at all. Also, if you don't trust the raw numbers, I'll use player efficiency rating (PER).
LeBron--18.3 PER
Howard--17.2 PER
Kobe--14.4 PER
Garnett-15.8 PER
Now for the guys who stayed in school and entered the NBA at a more advanced age:
Magic--20.6 PER
Bird--20.5 PER
Jordan--25.8 PER
Granted, it's a small sample size. But clearly rookies who enter the league at 20, 21, and 22 are more productive (assuming the talent level is similar) than rookies who enter the league at 18 and 19. Comaring Kevin Garnett and Hasheem Thabeet skews the argument a little bit because the talent gap is so wide.
For the guys who exceled in the league right away, going to college couldn't have hurt. But for guys who may be forced into the league prematurely because of factors other than talent level, it could potentially cost them an excellent career.
Don't look at LeBron, Kobe, Howard, etc. and think that's where the raised age limit comes in handy. Obviously, I'm more concerned about how it will effect guys like Andre Drummond, who will be tossed into the beast before they are ready to accept the mental and physical challenges that comes with it.