The moment we decide they're not good enough doesn't mean selling them prematurely. I don't mean sell the youngsters after a couple of performances, but I think we can agree that there are several players that we know won't make it here, yet we keep them in the squad for years. I think it was clear with Pellistri after he returned from Granada that he wasn't good enough. Yet we kept him another season, where he barely played, and then decided to loan him again to Alaves.
Did we really need to loan him a second time to determine he wasn't good enough? I know there are some late bloomers, but with the vast majority of great players, you can see it since his teen years. Players like Garnacho and Mainoo are the real gems that we need to keep and give playing time to. On a lower level, there are also players like Amad who may not have the potential to be world class, but you can see he has something in there.
I think a successful team needs balance not too trigger happy as you say but way more demanding that what we are currently doing. When was the last time we regretted selling someone? I can't remember a single player we've sold that I regret.
Now, regarding young talents refusing to join us, well, then so be it. If they don't rate themselves as good enough to make an impact, then why should we? The best ones will still fight for their chance, and those are the ones we should be keen on.