Well, Sancho isn't an established player already. He still needs to prove himself although he looked exceptional for Dortmund. However, he left so early you can't really say Guardiola didn't give him the chance to show his talent, do you?How about Jadon Sancho? He should have promised him enough first team chances. If you are good enough for starting at Borussia Dortmund, then you are certainly good enough to be the sub for the last 10 mins instead of 50m Bernardo Silva. He keeps stockpiling 50m players for the RW (he has three now), yet he had Sancho and Brahim Diaz, two very highly rated youngsters.
That is not to only criticize Pep on that matter, many top clubs are too afraid to play their youth at an early age. When did the last Juventus academy graduate earn a first team spot? What players did Barcelona, Bayern, Chelsea etc. pull up into the first team from their academy in the last decade? I can only think of Sergi Roberto at Barca and Alaba at Bayern. One first team player in a decade is such a poor record. No excuses for that.
Besides that I think the fluctuation of youth players is a general problem for top clubs right now. We never had such an insane concentration of top players as currently. The big teams just buy so many players for their first and second row and also the most promising youngsters. Yet there isn't a single top club which produced a note worthy amount of first team players in recent years. Of course they occasionally trust young players but those are usually exceptionally talented externs like Sane, Mbappe, Dembele, Kimmich, Asensio, Dybala and so forth.
With that in mind, it is hard to blame Guardiola for not trusting youth players "anymore". You could do it if there would be different cases but there honestly aren't any role models. Maybe Atletico but then again they do it because they have no choice.