It doesnt matter if I like him or not, my or anyone else`s sympathy for Mourinho is irrelevant. Facts are the way you pointed out - 2 UEFAs, 2 CLs, titles and cups in any of the 4 countries you mentioned. All that is undeniable. That timespan of 15 years can easily be divided in two halves, with 2010 being the end of the first and the beginning of the second, and it is pretty obvious which half has been more prestigious and successful. Everything remarkable Jose did, it was between 2003 - 2010, that was one of my points.
Again, facts are that he managed to win titles and cups in his second half, yes! Yet, it has to be said and underlined that he was at the helm of three clubs - Real Madrid, Chelsea and Man United - two of them top every single chart ever made about popularity, riches, fans (domestically, globally), sponsorships, TV money, turnovers, another money related stuffs and God knows what else... What he won in these three clubs doesnt look that exciting, compared to his previous achievements, it is rather disappointing and it is no wonder that he had been fired by all of those three clubs!
Can you agree that given the fact that his most successful period is between 2003-2010 where he managed inferior teams like Porto and Inter, his spell between 2010 - 2018, where he had all the money in the world in Real and ManUtd, is more or less a letdown? Not good enough, a fail? Isnt that some sort of proof that he`s declined?
His last 8 years, in his last 3 teams he managed, he already had star players in his teams, had excellent players, very good ones, he spent more than a billion dollars overall to buy the players he wanted to add to those who were already there when he took over, and what he did was win a single Europa League, one title in England, one in Spain and a few Mickey Mouse cups. That is not good enough for the money invested by those clubs, and his constant moaning, his excuses, his need to constantly remind everyone who dares to critisize him that once upon a time he won this and that are clear signs that he lost it. He cracked, he is too stubborn and proud to admit it, but thats what the facts show. He emerged, he peaked, then he declined.