Not everything is black or white (oh irony). All of Miretti, Fagioli, Rovella etc are promising players. It doesn't mean that they are going to become world class. It's even improbable that more than one will. In order for a player to become WC there is a line of factors that need to click. Talent, physicality, head, circumstances, surroundings and some random things that we wouldn't even think of.
It's a bit funny when people are giving a recipe for a world class player at each year of age. From a Juventus perspective though, we also need a host of just solid players to make the roster. Thanks to the recent youth policy it seems like we found a way to get those players much cheaper than paying millions to Sassuolos of this world. With a bit of luck we might get a very good or even an elite player from time to time. I see no point in criticizing single players like Fagioli, Miretti or Rovella just because they are not special or the club that it's not able to bring generational talents from Primavera.