Sometimes I can't change the manager's opinion...
For example (47243129) and (47243219)
I would not waste energy on trying to get managers to train HOF potential for U21. It's not going to happen. No good manager is going to train OD or other skills needed for a great U21 player and then try to also train for a great NT player.
Please target at most MVP but the sweet spot for U21 talent is allstar, perennial allstar, and superstar. These players will never make NT but can make an awesome U21 player if trained correctly, as they will hit their cap in their final U21 year or in the seasons just after.
My opinion.