We need squad players that’s why
We need squad players better suited to ETH's game, that's why he will ( or at least, Should) move, combined with his age. SAF also needed squad players - didn't stop him selling Phil Neville, or O'Shea or Brown when they got to a certain age and he couldn't offer them first team guarantees and thought it would be more effective to have younger alternatives happy to bide their time.
McT is limited when it comes to a progressive side, as opposed to a scrapping, counter-attacking team: he doesn't have 'big club' nous on the field- his best games have been where he was allowed to play like a cut-price Fletcher (without the same positional intelligence or clean tackling ability) allied with Gibson style striking technique, so Leeds or the Manchester Derby in Ole's last full season. He's much more use to a team like West Ham, and we can sign someone with better technique under pressure and positioning to fill-in for Mount or Case, dependent on the situation.
Edit: alongside also bringing through Mainoo, who can play a similar style with better technique and (hopefully) higher-grade situational awareness and anticipation. Even Hannibal, I think, could fill a similar squad role in 12 months time, hopefully after having ironed out some of his own flaws, cheaper and with with more composure. If not, then we should also look to move him on. No way Guardiola, for instance, would keep a player with Scott's limitations around indefinitely, squad or no squad, even if ETH's style differs in certain respects.