Holdbacks deprive those kids of appropriate age of spots at the higher levels.
Exactly the opposite of growing the game, holdbacks are effectively shrinking the pool of players.
While this may true nothing is going happen till the college coaches get on board. You don't know how many times a college coach encouraged a player to repeat or declare a PG year. Happens all the time.
If the people who will end up being their coaches are telling them to do it, what are they supposed to do?