What happens is that when a manager is not performing up to standard, and people want to defend their work and suitability, they will often argue that (almost) any other manager wouldn't do any better, and this is the squad's ceiling. This is a fine and sound argument, but it must assume that the squad also has a floor, and that a worse manager would perform worse. Otherwise you are not defending a manager, you are arguing that they are irrelevant, what JPRouve (I think) called a "glorified personal shopper."
The problem is that when you have Klopp or Arteta finishing 8th it's possible to imagine another manager finishing lower, because there's plenty more spots on the table. The argument is at least plausible.
United on the other hand are like 16 or 17 in the 'form table' since Amorim was appointed. A worse performance than this would mean outright relegation. I think most people believe that is implausible. So it leaves us to conclude that the manager is irrelevant, in which case it doesn't matter if they are sacked or backed or whatever.