McTominay is a regular starter for the team currently 2nd in the PL. Given his age, the amount of football he has played in his career to date and the position he plays, it is reasonable to expect him to improve a bit, but probably not a huge amount.
I think he could be a regular starter for a title winning team or CL winning team but is unlikely to be one of the best players in the team.
If you go back to the day before Bruno played his first game against Wolves last season, United were 5th, ahead of Spurs and Wolves on goal difference. So the team with Pereira, Lingard, Dan James, Fred and Scott, Ashley Young 956 mins, Mata 622 mins, De Gea making costly errors from time to time, was 5th after 24 games. On expected points, Understat had us as 4th at that stage of the season, as we were underperforming both our expected goals and our expected goals against. The addition of Fernandes, Greenwood getting regular starts, with Pogba appearing after the Covid break, took that team up to 3rd by the end of the season.
There seem to be a lot of posters on here who don't appreciate the amount of work that Scott and Fred put in, match after match, in the system we play. Of course you could replace them, but other players would have to cover far more distance than they have done this season.