maybe it's supposed to be kept basic and simple for the next generation. i mean... it's pokemon? i don't think it's one of those thing that's is supposed to grow with you. it's supposed to catch a new generation into merchandise submission.
*Edit* like blue clues or dora the explorer isn't supposed to evolve with you, you're supposed to evolve passed it.
From a certain point of view, sure. I don't think games should be subject to that sort of thinking, because games can be for all ages whatever their aesthetic.
If you find a game fun, then it can be a lifelong hobby/passion. No one really out grows games, in general. Humans have made myriad games, and have venerated and pedestaled many of them (Football, Soccer, Golf, Battle Royales etc).
The unhappiness comes when the sport inherit in the game is altered or reduced. It changes the game.
Although, unhappiness with a game can also come if it's in a series that keeps teasing innovation and new things, but then fails to deliver.
By growing up, games do become easier to play. Particularly anything involved on a table top or in a digital interface.
However, video games have the unique problem wherein part of their appeal are the things not directly related to the gameplay. They also suffer from the unique problem that different people have better or worse ability to interface with and play them. This generally results in the "dumbing down" of games, which changes the game enough to make it less fun for veteran enthusiasts. They also can suffer from graphical choice / operating engine choice impacting the gameplay itself, too.
Pokemon was initially aimed at kids in the 90s, and the premise was bug collecting, but with fantasy interpretations of animals with battles to give it an obvious, easily engaged with competitive aspect. A lot of small changes over the years have brought the competitive portion of its gameplay on stage, getting everyone in on effort values and the like, while the elements around the game have had little to no input, instead opting for weird mini games.
A ton of people love pokemon, and they just want a basic game with the ability to catch every single pokemon. There's like 900 pokemon now. Instead of doing this, though, they continue to steamroll out remakes and games similar enough to the old that you might as well still play any of the Gameboy color ones and have a superior experience.
So a lot of folks want something from the series, and they have the fear that they'll miss out on getting it if they put the series down. They also like to see what new things get added.
I think the TL; DR is, Pokemon fans love Pokemon and always will on some level, and they still want to have fun with the series. They can't anymore though, because it's become obvious that production of the games is not about creativity or fun but profit. The idea that they are only for kids is an excuse.