Thing with XYZ is that they were seemingly going for a high-risk high-reward type of series:
They made Ash more mature.
They upped animation to 11.
Writing felt somewhat more consistent.
Tropes not explored previously done correctly.
Main girl being romantically interested towards Ash.
Ash getting to the finals.
Ash actually having some kind of 1 in 10000000 thing that centered the entire region around him.
So they were going for pretty much the peak of what they could do with the anime up to that point, and it was working well up until the conclusion of several of these things being a complete 180 from what they had buildup for the fans, Ash lost the league in the finals, Serena romance plot obviously went nowhere, Ash-Greninja form was not ever implied to exist ever since, art style changed drastically, etc.
So it's not like it's a bad series or anything like that, because it's one of the best in the anime, but it definitely tried something too extreme when it came to hyping fans up for something that ended up disappointing most of them in the ending, I mean, the cope from "Down To The Fiery Finish!" is still as strong as ever.