I agree with most of what you said in this thread but I think, you are wrong on this. The goalkeeper is a major factor in you ability to choose the height of the defensive line. De Gea was as uncomfortable away from his line as it gets. Wasn't only him but also Maguire and Lindelof who more or less forced us to play deep in defence. If you have to play a deeper line, trying to press becomes way harder because you can't compress the space as good. Issue over the last 2 or so years was, that some of our attackers still tried to do it, hence it looked disorganized and sloppy. A press badly executed will set you up for hammerings.
Last night against Wolves, Onana played very conservatively in terms of positioning. Might have been a call from ETH to help him bed in better to the league (we've seen him getting lobbed already once). So I guess, we are going to see him push up a bit more and more and with him, the defensive line. Which then will decrease the space in between attack and defense. Hopefully helping Casemiro because he won't be on his own in midfield, and hopefully helping some of our attacking players to make more use of their pressing abilities.
The theory isn't faulty, it could work, but it is a big ask. For the record - I wouldn't have went for Mount either, probably would have gone for Amrabat to upgrade on Eriksen and then add another good midfielder on top, that can cover for Casemiro and potentially replace him at some point down the line. Probably would have went for Caicedo even though paying 100 million for him wouldn't have felt right as well.