Your idea of the older players dominating would work if incremental improvement were always guaranteed. If, for example, we could assume that a player would make an x degree improvement each year that he was in school, it would be fair to assume that a team of four year players would always be better than a team led by freshmen.
But incremental improvement is not guaranteed. Let's think of it like a video game. On most video games, players are rated at a certain number. Let's say for instance that freshman Perry Ellis was a true talent of 78 when compared to other college players. As a TT 78, he had some good games, but struggled at other times. He returns as a sophomore and is a TT 84 - he's improved quite a bit. Still some down games, but most definitely some strong games. So he returns again. This year though, how much better is Perry than last year's version? 86? 87?He's better, but only slightly so. And taking it further, how much better will he be? Does he top out at 88?
Now, take a guy like Anthony Davis. He probably entered college as an 87 or 88 on day 1, and was probably over 90 by the time his freshman year was over. In this exercise, senior Perry Ellis might not ever be as good as Anthony Davis was when he walked onto campus in Lexington.
And that's the best explanation in a nutshell. For the most elite players (because that's what we are dealing with in OADs) their ability is already higher than most players will ever achieve.
Look at an example closer to home. Ben McLemore and Travis Releford played at KU together for a year, plus an additional half year of practice. McLemore was, based on what a lot of people said in practice, often the best perimeter player they had. Even against a 5 year athletic wing like Releford, McLemore was pretty clearly better very early on in his career, and was far and away better by the time he started playing in games as a sophomore. If improvement worked the way your post surmises, Releford should have been schooling McLemore, but the truth is that McLemore is just a better basketball player than Releford ever will be.
If Anthony Davis, Michael Kidd-Gilchrist, Ben McLemore, Andre Drummond, Bradley Beal, Cody Zeller and Michael Carter-Williams were all still in college, they would absolutely be dominating. Same for Nerlens Noel, Shabazz Muhammad, Anthony Bennett, Gary Harris, Andrew Wiggins, Julius Randle, James Young, Jabari Parker, Aaron Gordon and a healthy Joel Embiid. But those guys aren't still in college. The vacuum that is left by them is filled by younger elite talent, not more experienced, less talented players.
Think of it this way. Michael Jordan was an exceptional player at UNC as a freshman. The only difference between then and now was that James Worthy was a junior, and Sam Perkins was a sophomore. Let's say that the OAD idea existed then. At the very least, that means that Worthy and his 15.6 ppg would have been playing somewhere in the NBA. Perkins may not have been in Chapel Hill, either.
But Jordan would have had a bigger role as a freshman if not for the fact that James Worthy was still there as a junior. Worthy, Perkins and Jordan were the only players that averaged double figures on that team (15.6, 14.3 and 13.5 respectively). Jordan actually took the most shots on the team that year. Worthy's 350 shots would not have gone to Matt Doherty or Jimmy Black (the only other players that took even 100 shots on the season for UNC. Those shots would have gone to Jordan and Perkins, pushing their averages up from 13-14 points into the 17-18 range. Jimmy Black would not have been taking shots from Michael Jordan, even if Michael Jordan was just a freshman.
The difference is that the elite talent is, for the most part, freshmen and sophomores now, versus being juniors and seniors 20-30 years ago.
As for the NBA issue, Adam Silver cannot make that change unilaterally. He has to do it in the next collective bargaining agreement. The CBA is set to expire in 2021, but could be renegotiated in 2017. Don't expect any changes before then unless NBA owners are willing to give the players a pretty sweet deal to come to the table earlier than they otherwise would have to.