You also need to look at Arsenal's spend, the more the club spent the more the team performances incrementally improved, because the players were being integrated into a system that was proving to be successful. By the 400 million mark they were challenging for the league.
You could argue that if Arteta was given a plethora of resources at an earlier point of convenience his football intelligibility would see a drastic development with his teams performances.
It's the exact opposite with Erik, he's such an inept manager tactically that despite spending 650m he's regressed to an even worse basis beyond the point which he was hired under Ragnick being 6th.
I don't think it's comparable to measure Erik hypothetically against other managers given the circumstances, it was the right decision to hire him on the premise of his perceived value but intrinsically he's catastrophically failed to deliver.
He's only got himself to blame under those parameters, I think most managers improve if given the resources and time he has, they might not win the league but the bar is so low with Erik that it's indispensable to sack him on the merit of the club's respect for itself alone, no other manager has ever plunged to such depths aside Moyes and we know his level as a manager.