No manager gets 100% what he wants. Sometimes the club disagrees with what they want, sometimes the manager wants players that don't want to come (or don't want to leave), sometimes the manager wants players that are just too expensive. Ferguson missed out on plenty of players, Guardiola missed out on plenty, Klopp missed out on plenty, etc. It's the managers job to do the best they possibly can with what they have.
We can't spend unlimited money, so the reason ETH had to make do with the likes of Weghorst, Sabitzer, Amrabat, etc on loan is because he wanted other players that ended up taking all the transfer funds. He seemed to be incredibly fixated on FDJ, a player that didn't want to come (he's also said that he wanted both an FDJ and a Casemiro, not an either/or). Reports are he wanted Kane, a player that Spurs would never have sold to us. A manager having unrealistic expectations doesn't mean he wasn't backed.
The managers have been working under a poor structure. There's certainly no denying that. However that doesn't mean the managers shouldn't be held responsible for the things that they can control. That structure is also being ripped up and replaced, so I'm not sure why you constantly repeat that if we sack ETH we'll be in the same position again in two years. We definitely might be, but only if we get a manager that isn't good enough. If we get the right manager we won't be.
If you agree that the four previous managers deserved to be sacked, then there is no relevance to bringing them up and saying that sacking them didn't work so therefore we shouldn't sack the current one. What happened with those four is completely irrelevant. We've replaced plenty of players as well, but that doesn't mean we shouldn't replace current players if they aren't good enough either. It's exactly the same.