XY didn't mirror AG at all though: Else Ash would have gotten Chesnaught (Sceptile mirror), Talonflame, Avalugg (Glalie mirror) and Clauncher (Corphish Mirror).
Pikachu = Pikachu
Greninja = Sceptile: Fully evolved starter.
Talonflame = Swellow: Fully evolved bird.
Hawlucha = Corphish: Powerhouse that did not evolve.
Goodra = Torkoal: Would-be powerhouse that only has one or two wins. Remember that Ash caught Torkoal straight off the back of facing Flannery's
impressive one?
Noivern = Glalie: Late capture that evolved.
But that's just the pokémon side of things. How about the younger sibling of one of the main characters joining the journey? The last time that happened was.. Well, AG. So how doesn't it mirror AG at all, when one of the core-features of the series is the first time they've done it after AG??? Also, AG was very focused on battling, as was XY.
The biggest dissuasion from the comparison is that SM drifts from the classic formula greatly, while DP suffered from fatigue of it (so much the following series started deviating from it in significant ways). Also while DP played off of a lot of the previous series, AG's ideas, SM, for better or worse, discards a lot of XY's structure to do completely it's own thing.
There are some things that do stick out (a female companion getting heavy limelight, Team Rocket having a more dynamic role while also being ten times wackier, drifts between very light hearted and very dramatic) but not enough yet to make it a true counterpart.
I actually don't think that DP suffered at all. There was a lot more individual focus on the pokémon, not just Infernape, but even stuff like Paul's Torterra, Dawn's storyline was undeniably better when compared to May, even when the latter had TWO Grand Festivals - as was her status as "co-star", and the rivalry with Paul was dramatically better than ANYTHING AG had shown us in terms of rivalries. The only fatigue that might have been there, may have had something to do with Brock - as he did leave at the end of DP, but the structure of the series as a whole reached its pinnacle during DP.
Best Wishes didn't switch it up all that much. All it changed was change up the cast by ditching Brock and Dawn (the latter part was expected), introduced rotation for Ash's team and stumbled on pacing. If anything, Best Wishes is when the fatigue started setting in: It tried to emulate OS
and DP in a shorter series, by giving Ash more pokémon like he did in OS - which meant that there was less individual focus on the pokémon as was the case in DP, the characters also turned out bland and weren't developed fully, Cilan never did anything to grow as a Sommelier and Trip is best forgotten as a rival and the league turned out to be the most disappointing one yet. Highlight for me was Iris' development going from clueless girl with a Excadrill that wouldn't listen to self-assured tamer of a rogue Dragonite - with several key episodes dotted along a rushed series to tie her story together.
To sum it all up, I was excited when the Sinnoh League rolled around. I couldn't help wondering "just get it over with" when the Unova League rolled around. Which makes me pretty sure that "fatigue" set in during Best Wishes, and not DP. Which makes XY such a remarkable series that bounced back from that series. Sure, XY has flaws, massive ones. But it did try to focus on characters yet again. They took their time with Serena, before throwing her into a career with an unfounded choice. The best part was the Mega Evolution Acts leading into XYZ, which was great in its storytelling.