Still too early to judge kids like Iker Bravo or Aznou but safe to say that most of those who have left and never returned (so obviously excluding likes of Pique and Alba) never had quality to make it here. Ofc it would help if we would actually sell instead of letting most of them go for free but OTOH mostly they left when club was still in a good financial situation so getting a few more millions in the bank back then wouldn't help us all that much right now.
Olmo (who could still return some day), Icardi and maybe Onana made the best careers outside of Barca alongside Cesc who was great at Arsenal but not so good when he returned. Jury is also still out on Sergio Gomez but I doubt he'll become a starter at City and with Balde now fully established it's not like we urgently need another young LB.
The worst one is Pablo Moreno tbh.
I would say plenty of them did well for themselves and would have at least got a promotion to 1st team but wouldn't be a starter here, guys like Bellerin, Balde etc.
Some simply needed that extra moves to develop, Gomez is prime example as it was that move to Anderlecht that finally made his career since he became a LB there. Xavi Simons, Olmo and maybe Moriba too. All had to experience some sort of humbling experiences before reaching good heights. Don't think any of them would be world class player, but probably will get a good career.
Overall not a bad list of La Masia graduates