There are two sides of this question. First of all, he can develop, many good players were nobodies back when they were 21. And the fact that his unexpected starting role did not faze him last year says a lot.
But that is not what ultimately important. If you wanna have a real dynasty a successful club, like we had under Fergie, you need players like McTominay. You can't buy the whole squad, 24-25 players. You can't have so that every one of them is good enough. A real trick is not only to have a stars, but also a solid back ups. Fergie used to have that with Butt, OShea, P Neville, Fletcher (he had some great years, but was largely average at the start of his career and after injury). Those people were nowhere near as good as Keane, Scholes, Rio or whoever. But they came on, when we had injuries or players were tired and they gave 100%. And seldom they let us down.
And the thing is, it's very hardly to buy a player like that. Actually probably harder than it is to buy a star. Because you need a player that essentially would be ready to sacrifice big portion of his career to be part of the club, understanding that he will not play as much as he could have. And usually home-grown players are best for that.
So i think while it's possible McTominay will never be truly United's level footballer, he can be a part of a successful team, maybe for many years to come.