It's more about succession. We change managers and, at best, only marginally improve.
There must be something else going on in the club's overall structure for this to continue, as it has.
Yes, and probably the single biggest part of that is that the club hierarchy chose the wrong managers. Certainly not the only part and even someone like Pep or Klopp would have had serious problems, but we could have had the best set-up in the world and the managers we did have ultimately wouldn't have been good enough (although they may have won a few more cups here and there).
Moyes clearly wasn't good enough.
LVG was past his best and wasn't going to be good enough himself, but he did set in place some building blocks for the next manager to take and improve on further.
Mourinho was past his best and basically the exact opposite of LVG, so any benefit we got from the Dutchman was ripped up and discarded instead of being built on.
Ole wasn't good enough.
ETH was the first and only time that we signed the right profile of manager, but it's now almost certain that he isn't going to be good enough.
The idea that we've been changing managers and it hasn't worked so therefore the managers aren't at fault only works if you think any of them actually would have succeeded with a better set-up above them. In our case, none of them would have.
We've signed the correct manager once. It hasn't worked which is unfortunate, but managers who are doing good work at smaller clubs often fail when they step up. It's not a big deal. You just sack them and hire another. Keeping that failing manager until somebody comes along who is almost a 100% certainty of succeeding is the worst possible thing you can do, not least because a manager of that pedigree will have much better offers than a club who has shown such poor decision making. By keeping a failing manager and allowing him to keep failing we are actively making our club less attractive (to top managers and top players), and makes it that much harder to turn things around.
The next manager we sign does not have to be the one to take us back to being one of the best in the world. Obviously it'd be great if they do, but the more likely scenario is that they will be a stepping stone. They set in place the right tactics and style, they improve things and develop the players, and just simply move the team as a whole into a much healthier position. And the fanbase has to learn that even a manager who does all that, may not be the best for the longterm and should ultimately be sacked if they've taken us as far as they can. Which is going to be difficult for the fans, considering how much support the manager who took us to our worst ever league and CL performance somehow still has.