He's a kid whos played 7 games, half of them hes not played well.
You could go for most midfielders in the top 5 and they would be better.
I don't think you understand roles and profiles. Just because there are a number of older, more established and better midfielders in the Premier League, it doesn't mean that they can do every role in the midfield. KDB wouldn't be efficient at #6, and Rodri would be useless as a #10.
Mainoo would start for Arsenal because they lack a player of his profile. He starts for us. He'd get game time for City because they didn't adequately replace Gundogan. He would get a lot of game time in every top 5 team right now.
Judging by your logic, Rico Lewis at 18 wasn't an important piece of City's success last season, but he was.
Saka has been one of Arsenal's most important players since he was 18.
Garnacho has been important for us even with Rashford occupying his preferred position for most of the time.
Zaire-Emery has been crucial for PSG at 17. Lewis Miley has played important games for Newcastle at 17.
Gavi and Pedri have been crucial for Barca ever since their teenage years as well.
Why would you sell a potentially world class, homegrown talent that's such a unique, rare, and highly sought-after profile when you are literally one of the richest clubs in the world, which United definitely are?
I can guarantee you that all of United, Arsenal, City, etc. would refuse 100m offers for Mainoo without much thinking.