I don't think that's "proof," no. I think it's one example where a player improved between the ages of 18 and 20 by staying in school and another player may or may not have improved between ages 20 and 22 by playing against pro's. You're right that there are tons of examples and counterexamples of guys who improve early, and other guys who improve later, for all sorts of different reasons.
But here's the thing: it's really easy to forget that these guys who many write off as "old" or that their upside is limited or even expired due to their age are, like, 20 years old! That is not an old basketball player. In most cases, that is not a fully developed basketball player. It wasn't all that long ago that all 20 year olds were still college juniors and several years away from being ready to contribute meaningfully in the NBA, and it's still true that many aren't close to ready at 20. Sure the top players are, but many aren't.
I think about it this way. The average NBA player is at his peak performance, the height of his powers at what, 24-25 years old? Give or take a year or two depending on the player. Some might say even older than that. So an 18 year old has, say, 7 years until he reaches his peak and a 20 year old has 5 years until that point. Again, on average. Is there really THAT much of a difference in a team "only" getting a guy for five years pre-peak as opposed to seven? I don't think so. How many guys even stay with their first team for more years than that?
Seems to me there should be value in the 20 year old -- like Wendell Moore -- who a franchise can see already HAS shown the ability to improve and to develop, which often indicates that there is room for more development and the willingness to put in the work to do so, as opposed to a guy like Keels who, while younger, it's mostly hope and prayer that he can develop his game. Yes Keels (again, just an example) theoretically has more time until the typical peak of 25, but he may not have the ability to improve at all no matter if he has two years to do it, four, six, or the rest of his life. I think about a guy like Trevon Duval. Tons of natural, athletic talent and ball skills. He had lots of "upside," meaning at age 18 or 19 coming out of Duke, if he could develop a shot and develop better playmaking and decisionmaking skills, he could be a star. At least some thought so. But he didn't (apparently) develop any of that, and he's nowhere in the NBA right now.
So I guess that's all a long way of saying that I'm not convinced that the conventional wisdom is right. In some cases it is, sure, but I don't see a consistent evidentiary basis for what for some are blanket statements about this 20 or 21 or 22 year old being too old or having his "upside" limited while this 18 year old has "huge upside." The whole thing is much more complicated, and much more individualized than that. Of course, I'm not running an NBA franchise, but I wonder if there is some groupthink going on among a lot of these guys -- that's another way of establishing a conventional wisdom. Doesn't mean the reasoning behind it is sound.
Right but the issue is comparing the 22 year old who has produced xyz to the 19 year old who hasn't produced xzy and many times hasn't produced close to xyz. The NBA valuing the 19 year old in that situation, based solely on his perceived "upside", while sometimes it's going to work out, seems like a pretty risky bet in a lot of situations.