Stones and Keane are PL level players at least, good in their own way but too early to tell how good they could be. Stones is not a natural defender but will get a lot better with experience. Keane is a proper defender in the John Terry kind of mould, pretty good with the ball but not as good as Terry was and less of a cunt.
Holding and Tuanzebe are nothing but potential at this stage. Just like a hundred other young English players who barely made a dent. If you name United's best 10 youth players (that they produced) since the golden generation it is nothing to get excited over. The best have been Darren Fletcher, Wes Brown, Keane (with Burnley), Ryan Shawcross (Stoke), Danny Drinkwater (Leicester), Joshua King (Bournemouth), Danny Welbeck and now potentially Marcus Rashford. Some good players but not many likely to put fear into opponents at top international level, which is surely the entire point of the statement "the future is bright".