I dunno because there are an awful lot of people than have been unhappy with pokemon for a long time. And everytime a new trailer for a new game comes out I feel like more and more of the fanbase gets annoyed.
Then again the problem continues because most of those annoyed fans always end up buying the games so it kind of defeats the purpose.
It is what it is i guess.
I dont agree with your idea that the stagnancy is one of the reason for the success, because there are a shitload of spinoff pokemon games that play nothing like the mainline series that sell almost as well as the main games. Ranger, Mystery Dungeon, hell look at the hype for a new Snap. Pokemon fans are addicted to the Pokemon, not the bog standard JRPG's of the mainline series.
Its a little of both for me. I like pokemon and I like seeing the new stuff - Snap may convince me to buy a Switch -, but when it comes right down to it I buy main line pokemon games for the same reason I buy games like Minecraft, Terraria or Satisfactory. I am here to proceed through a low stakes series of events where I will tailor my preferred system (be it house, mining setup, factory setup or team setup) to my own desires and then trudge forward through relaxing monotony and familiarity until I hit the end having only been challenged once or twice the whole damn time.
I was legitimately pissed when I bought Moon and realized that it is 3D and the camera is different, because I was seriously entering into the situation with the expectation that I would get the same game as the something like six or seven games I played leading up to gen 5 or so. I'm still going to play it, but I honestly don't want to see innovation in main line games, I'd rather they save it for spinoffs and side games.
I know that seems backwards and silly, but I kind of think that's the mindset of a lot of pokemon fans, and part of why the franchise as a whole continues to see success even though when you step back the whole thing seems all over the place. I cracked open the wiki article on pokemon to see what generation it was that I stopped playing (lined up with a very distinct date in my life, who would have figured) and I'm looking at the spinoffs per generation and it is nonsense. After the first two generations every new iteration has like five or six spinoffs. That's insane - the series is criminally unfocused, but it shows you where all their creative and innovative energy is going right? And it explains why people can still find fresh things to enjoy about pokemon in spite of it being two and a half decades old.