His way of playing football used to win him trophies but now even second rated coaches and teams have figured him out. On top of that he is not a respectful person either and therefore cannot command the dressing room of a team like United. He should just fucking retire.
It's not so much about coaches having figured him out. What he was ahead in were three things: Motivation, Preparation, Defensive Organization.
Motivation is gone because the young players are not the same as when he started 17 years ago and this generation is different. He can't reach the players anymore and they see through his methods and consider it a cheap trick. So that is gone.
Preparation does not stand out anymore as the methods he used in 2004 are now standard and even IPL coaches do it. So no advantage there either.
Defensive Organization has also moved on from his time. Now the focus is around the ball and what to do with the ball whenever you have it. Something he doesn't train. His defensive organization is still the same as 17 years ago but football has moved on.
Basically Mourinho is a dinosaur and his football and methods are antique.