You are actually right in everything you say but the human body does not respond to stress in that way. Just because they get much more time than others doesn't mean they won't be under constant stress. Either way you are right. No other top club sticks around with mediocrity as long as we do. This board doesn't even understand the word ruthless.
They gave him another 3 years after losing a cup final to Lord Emery's Villareal...
It is nowhere near the most demanding job in football. In fact, it is the least demanding job from any of the managerial jobs in top clubs.
Essentially, there is no other big club who gives near as much support as United. Unlimited signings, highest wage bill, fantastic fan support in the stadium. All you have to do in return is get a place in UCL, play shit on a stick football and smile, and you can get years as manager, while on any other big club you would have been sacked within the first 6 months.
It's SAF's shadow.
Someone like LVG and Mourinho with their own way would outright clash with the United way ( romanticized fond memories about SAF). They would try to imprint their philosophy to full extend, but would be opposed. In the end, they fought in vain to convince the fan base that they could bring some successes with a different way. They failed to build the team to their likings.
Moyes himself is a better coach than Ole, but he doesn't know this club of this size. He's completely out of his depth in every aspect. Immediate successor for SAF of course even hampered him more.
Ole knows the club better than the other 3. This helps him in emulating SAF to a certain extend, and in that he has gone the furthest in term of squad building. This team on paper wouldn't be far from what SAF would build from the same circumstances. However, SAF himself would have done much better, while adding Ronaldo, and Varane with the expectation coming with it, is looking more and more a curse on Ole. Ole can't no longer hide behind the rebuild. He's expected to deliver result which comparison to SAF's standard is inescapable.
Chelsea, Real Madrid, Barcelona... may be more demanding in term of instant successes, but you can feel and see that they're clear with their aim/pulling in one direction: win or out. There models mean they would go through many managers quickly. What his mean even if the manager fails, it doesn't mean he wouldn't suit other clubs. Just his bosses have to save his own asses, and have to act quickly,
Here our manager job in reality is to fill the void SAF left after his retirement which would include both result and vibe. While this club is not as ruthless based on result, it's actually more tormenting for the managers awaiting their slow death (of their reign). There is no accountability for failure from the higher up, so there is no mercy dismissal, but leaving the managers to dig his own grave trying to escape SAF's shadow all on his own.