Is it? in terms of single player games maybe, but games like Smash, Pikmin, Kirby, Mario Kart (and other spinoffs) are just as, if not more iterative. Nintendo loves their formulas, and I don't blame them when they're successful on that level. Zelda definitely was on that static formula before BOTW.
Smash and Mario Kart evolve as much as they can within their respective genres. It's true they don't reinvent the wheel each time, but I don't see how that's even possible for a Kart racer and a 2D fighting game. They are still evolving from a technical standpoint and include various new gameplay mechanics and lots of content with each iteration, while Pokémon is cutting back on actual meaningful content for generations and is replacing it with more and more gimmicks instead.
It also doesn't help that Pokémon has become an iterative franchise, while Nintendo makes sure to release only one Smash or Mario Kart per generation.
With Zelda and Pikmin I don't see your point at all. There are a lot of changes between Zelda and Pikmin entries. Of course up until BotW the underlying gameplay mechanics have been the same between all 3D Zelda games, but the series has evolved just fine. BotW was a revolution of the series, not an evolution. Pikmin on the other hand only got three (mainline) games so far in over 15 years, which is not enough to become stale, even without evolving. On top of that each game offered two new Pikmin types and in Pikmin three you could control up to three characters at the same time, sending them each on their way to fulfil specific tasks, which really deepened the underlying core mechanics and added another layer for players who want to manage their time as efficiently as possible.
I wouldn't even agree on Kirby. Even though they are releasing a lot of Kirby games, they are always experimenting with the series (Canvas Curse, Epic Yarn) and with the classic Kirby platformers they at least introduce interesting new gameplay mechanics with each game (several layers in Triple Deluxe or the robots in Planet Robobot).
Honestly, the Nintendo franchise which comes closest to being as stale as Pokémon is probably New Super Mario Bros and even there it's purely from an artistic point of view.
it still followed the linear dungeon to dungeon style via a hub field with an instrument and sidekick to get some number of relics and featured recurring characters and items+weapons.
it was formulaic to the point where the series had a massive change that it was praised for?
Most of the gameplay was still finding small keys to get the dungeon item to get the big key to fight the bosses.
And still in terms of atmosphere, storytelling and gameplay focus, there are lots of differences between the 3D Zelda games. Pokémon games are all about catching 'em all, becoming the very best, stopping the evil team of the season and beating eight gym leaders. No one is asking them to get rid of that. But just as Breath of the Wild or Super Mario 3D Land were trying to refocus on their series' origins and retroactively incorporate design elements of the 2D games into the 3D ones, GameFreak have to take a step back and look at what made Pokémon appealing to a lot of kids in the first place. For me at least, the feeling of going on an adventure in a (seemingly) open world is a big part of that, which has been lost thanks to their increasingly heavy focus on boring story-telling and linear level-design.
I don't at all actually. Pokemon is about accessibility to anyone of any age. The current formula is perfect for that, an open world is not.
Open world games can be accessible to all. Heck, the original Pokémon games were much more open in design than modern ones and they were still primarily played by kids.
it's cute that people imply that GF doesn't get it when they are probably the absolute best in the business at keeping a franchise alive and kicking over such a long time.
McDonald's is a multi-billion dollar company, operating almost 40,000 stores in 120 countries worldwide. Doesn't make it good.