Look, I also wanted him sacked in his last "3rd" season with us. But not because he was a bad manager, or because of his tactics. I wanted him gone because it was the only way to go forward.
Mourinho builds trust with his players and when everything goes south and the trust is gone, it can't be recovered, because he is a perfectionist. I remember wondering at that time, what if the board has the balls to get him back in the summer after selling the players he fell out with and recoup their value to reinvest in his targets.
All his 3 seasons with United were very similar to Chelsea's 2nd spell. The first season was about assessing the team and getting some leaders in like Ibra, or Etoo in Chelsea's case. 2nd season is his punch season when he knows the team and knows what to get in to improve it fast and compete. You would have won the title in the 2nd season if there wasn't for City with their stacked squad in all departments.
The 3rd season was a bit different for United compared to Chelsea. At Chelsea, Jose and the board believed too much in the team after winning the title so we got only Pedro in that transfer window. The rest was history, key players came back unfit resting on their previous season's laurels, Jose went under immense pressure from the media, and it was just a cloud of negativity with no exit. He was sacked late, which means the board understood the circumstances, but in the end, had no choice but to sack him.
At United, the 3rd season should have been about another try to improve fast the team, similar to the 2nd season, with a shrewd transfer window. And I believe the board wanted to do that, that's why they agreed on a new contract, but then failed to deliver on the commitment after seeing the inflated market for key positions, as well as the age of some desired players. And here was, in my opinion, the big mistake made by the board. Chelsea also did it, with Antonio Conte, and it seems that was a lesson learned because in similar circumstances they decided to part ways with Sarri, instead of delaying the transfer ban to get his targets.
You see, a manger, with all his experience, has a vision of a team and what it needs to perform in accordance with his philosophy, and what both United did with Jose and Chelsea with Conte isn't acceptable, you either commit, or you don't. And before some of you mention the age of some desired players Jose wanted and the fact that those players would need replacing in the future, you forget how important experience is. Look at your team now, there are no leaders, it looks like a kindergarten squad with kindergarten mentality across the board. Who are the young players looking up to? Pogba?
P.S. sorry for the long post. I just have limited numbers