It's a hit or miss, but I don't find it any more difficult to convince people to train lower potential players. I've seen a few all-stars come into the U21 in my current tenure, and even more in my last account (not including those which had asterisks haha). Plus, I've also come managers who despite getting talents that are MVP and HoF, and declined to train them properly or resell them (I don't even know why they bothered to buy the player in the first place).
I'm pretty sure you know very well, having taken care of micronations, that our rosters are highly reliant on what is drafted. If you look back at the match history of our U21, you'd find out that last season, we had quite a few 21 year olds who were below 9 potential, while the season before that, we had a lot of MVP and above potentials. Season 51 however, saw majority of the players have less than MVP potential. So rather than meeting more people who only wanting to train high potential players, it's more of a "I'll take what's available".
But as to why high potential players still train into U21, you should reread the paragraph you quoted:
The players in the Future NT player group however, while they are trained in ways that allow them to qualify for the U21, but once they hit 21, their training plans will be more focused on allowing them to have a smoother transition into the senior NT.
We haven't reached the stage where we can totally save MVP and above players for the senior NT while all star and below are solely trained for the NT. Therefore for these players, I'd need to strike a balance between their final product. So far it's been working pretty fine thankfully.