Not wanting to learn from the greatest manager and great players like David Silva, KDB, Aguero sort of contradicts what you say. Foden was nowhere near the first XI when Sancho left and is now better than Sancho. Because Foden has learnt a lot from working with the best manager and some of the best players.
It's important for young players to play, but it is also important to learn from the best, and Sancho snubbed that chance. Think he'll pay for it with a shorter and less exciting career at the top. He will have good seasons, perhaps even at United, but won't be among the very best. IMO, of course.