I don't disagree with what you've said fundamentally. However, there's two big missing caveats, one being we don't have enough quality on the ball available and that ETH has shown he can't get the best out of our midfield or attack. I'd be extremely surprised if he plays any four of the following at the same time long term: Casemiro, Mainoo, Mount, Bruno, Collyer and McTominay whilst keeping at least one striker and another wide attacker.
So realistically Bruno/Mount are always going to play at the 10 and you will have two midfielders from the above behind, with 3 attackers ahead. Casemiro and Mainoo don't work and I doubt Ugarte and Mainoo will either. Again it's not necessarily Mainoo's individual quality in question, it's ETH's coaching and the teammates around him; I will say add though that Mainoo is far from the finished article and we don't need to treat him as an indispensible on the pitch as of now. Therefore, in my mind it's all about damage limitation and playing to your strengths. Strong base, transitional, counter pressing and hard working but functional in the middle. WIth Martinez, Dalot, Mazroui, Casemiro, Ugarte, Bruno/Mount and Amad dropping into the spaces, there's still enough technicality and ball receivers in the tighter spaces to create vertical play. Hopefully de Ligt will add to this too because Maguire is clearly not it on the ball these past two games.
Again, I loathe that it's come to this but it's the most practical. ETH can rely on his defensive, attritional but progressive type of football without needing to coach the expansive pass and move stuff that he's not shown in 2+ years. Mainoo would be the 'casualty' in my scenario but it's absolutely not the end of the world as I see this as a short to medium term thing. Depending on how you see it or not, the most important thing now is the results.