Part of my desire for a high school diploma is that I want these kids to go through 4 years of high school coaching but also make them know that you need to do more in life than be good at basketball to go on in life. For every Dwight Howard and Kobe Bryant and Kevin Garnett there are thousands and thousands of almost talented enough for pro ball kids that think they don't have to go to class or study or anything else because they can play ball. What happens to those kids when they find out, they aren't good enough for the pros?
This way if it is a requirement for NBA employment and these kids know this at 12, 13 and 14 as 8th graders and freshmen in HS while AAU and HS coaches and agents are already starting to whisper in their ears about future greatness, they will know that without a doubt they will have to finish HS, at the very least.
And if come their senior year they start to find out they are just on the cusp, having that HS diploma gets them into colleges where further education and opportunities, even basketball opportunities can be created.
I would definitely demand a HS diploma be required if the age returns to 18. Maybe not for the good of those that are good enough to make it, but for the good of those that aren't.