I disagree, it’s an accumulation of factors.
Better players up front means our attacks don’t breakdown so regularly through misplaced passes, putting central midfield under so much pressure as they scramble to transition back into defence.
Better players means being able to press higher and break down attacks higher up the pitch, relieving pressure on the central midfield.
I agree we need another specialist DM to really make it work against strong teams, but for my money Luiz is more than good enough to fill one of two positions.
It might mean SJM having to work harder on that side of his game and that’s fine. If he’s on the bench, fine. We’ll set up differently for different opponents, we desperately need more tactically flexibility and the personnel to enable that.
Finally, in terms of defensive play I remember watching Jack track back from the opposition box to our own, win the ball and then carry it upfield again. I don’t buy the argument that he doesn’t put the effort in defensively. No he won’t win aerial battles in midfield very much, but that’s not really his job - imo.
Edit: also, Edouard. Great player!