Anybody who knows football knows the new manager bounce phenomenon.
While it's true that the "new manager bounce" is a real thing, I wonder how much it would really apply to an interim manager who, as far as the players knew at the time, would only be there until the end of the season.
Now, I don't necessarily think Ole had it in him to be a successful long-term manager at United; but aside from Moyes, we've done quite well every time we got a new manager. Granted, LvG and Rangnick didn't have a particularly long "new manager bounce" phase, but if we look at Ole, Mourinho and ETH in particular, we did well for quite a while when they arrived. Then it fell apart after about a year. That's a long bounce.
But then we also see the same with players. They arrive, and usually they do well for about a year. Then their form vanishes without a trace and they become shit. Not
every player does well upon arrival (e.g. Mount, Donny, arguably Sancho although he was at least better in the beginning than later), but it's definitely a trend. It's a trend that encompasses the whole club: new arrivals do fine for a while, but then as the new guy veneer wears off, it all falls apart and they seem to become mired in the same shit that engulfs the rest of the club.
For that reason, I have a hard time accepting the idea that we've just been appointing one useless manager after another, and signing one dud after another. Certainly there have been some where it seems clear that we did in fact get the wrong guy, but can it really be true that this was the case just about every time for a decade? That seems unrealistically improbable.
Seems more likely that there's something toxic going on at the club that prevents anyone from doing well in the long term. They tend to do fine when they arrive and then the thing, whatever it is, wears away at their capabilities. While I can't sit here and pinpoint the exact problem with total certainty, I think it spans all aspects of MUFC, from the ownership down to the general atmosphere at the club.
The state of the stadium, having to work under the parasitic Glazers, answering to people appointed not for their footballing credentials but for their business world resumé. Even basic things like us apparently being (up until relatively recently) the last PL club to not pay its rank and file staff a living wage. These things all contribute to the way people feel about playing for United because the psychological aspect plays a huge part of football, and if you come into work every day and are met with grumpy, unenthusiastic staff and a stadium that's falling apart, I'm convinced that these sorts of things can have a knock-on effect that can be seen on the pitch.