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But how is he one of the most successful coaches? I don't take into account his titles in Qatar here as this can really not be compared to anything coaches win in European leagues.
Well, this is where your mistake lies, you are too focused on European competitions and don't put success outside Europe in a coach's travel portfolio.
It would be a joke to argue that because of those titles he is one of the best. It is now his third year in Barcelona and he won La Liga once and the Super Coppa once. Barcelona got eliminated as third in the Champions League group stage last year and then got eliminated from the Europa League. Maybe you can provide some info as to why you think he is one of the most successful coaches. Compared to which other coach?
In La Liga, there are 20 teams with 20 different coaches. Now tell me who has won the La Liga title apart from Ancelotti, Diego Simeone and Xavi in the last few seasons? If you don't find another coach other than the three who won the title, then Xavi will be the most successful coach compared to 17 other coaches.
Ancelotti and Simeone are two more successful coaches in Europe than Xavi, so of course I'm not going to argue with that. After all, Xavi didn't come to Barcelona when Barcelona had great players and was at the peak of glory, while Ancelotti had everything to win something.