I think Sir Alex was also part of the problems that befell Manchester United when he resigned. News then had it that he was the man that handpicked his successor which was David Moyes. That was the genesis of Manchester United problems, an inexperienced coach handling a team like Manchester United at that time, this was a title winning team that was handed over to him.
You may be right, but every team has had a bad coach before. Even the most successful teams in history, like Real Madrid, Barcelona and Bayern Munich, have all had bad coaches. You can't tell me a single bad coach as far back as 2014 is still responsible for how horrible the club is now. At the point, there were too many factios at the club and so many things wrong in the club. What they could have done was deal with what was wrong, before talking about a new coach.
They could have gotten a good coach and allowed him to choose his player, handle his team the way he wanted and deal with stuff the way he wanted.
People often talk about how Manchester City is only succesffus because they spend big, which is not true. United have also spend as much during that time. The difference is manchester United have a good coach and he is allowed to handle his team the way he want, the same Ferguson was allowed to do. He request for the kind of players he wants, he tells the club the ones he wants to be sold, he promotes the academy players he wants, he deals with the dressing room and atmosphere of the team innhis own way.
We saw the same with Klopp at Liverpool. For a club to be operated as United where the coach has limited access, then every department of the club has to be very good at their job.