Anime episodes have a six month turnaound which means this would have been finished being worked on back in February. Realistically speaking, this would have been planned from either close to the start of this saga or even as far back as when XY&Z was still airing. Factor in that this year is the anime's 20th anniversary and it becomes very clear that this is a continuation of the celebration that Movie 20 started.
This isn't some spur of the moment thing or ratings stunt; they had this planned all along. It has nothing to do with fans being bored of Alola. Not that fan reaction matters for a show that gets a new audience every generation.
As much as I want to agree with you, the fact that the Anime is doing very poorly in ratings right now indicates it actually gets less of a new audience every new generation, even as early on as AG. And it also doesn't help that Movie 20 didn't even bring Misty and Brock in, and they definitely would have at least known about fans' outrage in leaving them out of the film since they were swamped with Twitter posts denouncing it.
Besides, technically, the change in animation would probably lessen the whole development period, anyways.
That is what I think as well. The writers only care about promoting Kanto and no other previous-gen regions, hence causing a lack of continuity with AG/DP/BW/XY. I know when the writers make bad decisions which won't resonate well with fans, and the promotion of Kanto is clearly becoming a huge problem.
The games overdosed with Kanto pandering, such as the player character's old home and the Alola Forms all being Kanto Pokemon, coupled with 1st Gen taking up the most slots in the Alola Dex, ahead of the 7th Gen Pokemon.
Eh, at least the anime's giving more focus to Kanto and Ash's journey there anyways this time around. The last time there was an anniversary for the franchise was back in Battle Frontier, and they barely even acknowledged the events of Ash's first journey in there (other than the first few episodes of Battle Frontier and the flashback episodes for Charizard, Squirtle, and Bulbasaur's return, when have they ever done a direct call-back to Ash's journey in Kanto? If anything, they treated it as just another region). If they're barely even going to acknowledge Kanto at all despite him travelling through it, well, it's better off if they just travelled through a different region altogether. Compared to Battle Frontier, this is tolerable, since they're actually giving appropriate focus to Kanto since, you know, he's actually travelling through it soon. I'd also expect the same with Johto and any other region he returns to as well.
Besides, even if Misty's return was just for nostalgia, as long as it means Misty actually returns to her Water Pokémon Master goal for good and doesn't result in her being shoved back in the gym once this is over, I'll take it. It was already very stupid of the writers to just cast off Misty like that. They could have had her decide to try to train for her goal by herself, or heck, maybe even come up with the idea of becoming a Gym Leader by herself and volunteer if that's what it takes. But no, they forced her into the gym via her sisters, and they did so TWICE. Misty is one of only two characters to really get a crappy send off when she left, the only other being Max (and she had it worse than Max because unlike Max, who really shouldn't have been on the cast due to being underage and then there's the whole aversion to having the cast actually age, so a broken promise was the inevitable result, Misty was literally forced off the show in ways that actually could and should have been averted). And believe me, after how the writers had her forced back to the gym twice (GCYL and A Real Cleffa-Hanger), not to mention DP setting up a potential return ultimately to fail to deliver it at all, didn't even bother to bring her back beyond Charizard's flashbacks in Best Wishes DESPITE the existence in the games of that Pokémon World Conference and that actually posing a good way to grant actual closure to her at the very least due to the show not even bothering to focus on the Pokémon World Conference, she DEFINITELY needed that return, nostalgia factor or not.