It's preseason. Results don't matter much. Let the kids play. I agree that it should be more structured so that the Juvenil A players get to play with senior players at any given time.
Gabriel Jesus -> Pep
Davidson Sanchez -> Playing time, Spurs has an ex-Ajax contingent
Mbappe -> From Paris, $$$
Keita -> Klopp
The players you listed moved to the clubs for reasons other than the prestige of the club alone.
I think people are excited to see the youth prospects play well. It's clear he's promising, but the fans eagerness to see them do well is the reason behind the exceptional praise.
And what if Digne wants to leave to go somewhere he can start. Somewhere he can play regularly and get back into the French team?
The club has a history of letting go of players who want to leave through the proper channels without making much of a fuss.