The thing is with Pokémon they've probably had ideas for years that they didn't use, but maybe do in the future. This could be anything from new forms of battling, new mechanics, new Pokémon including evolutions or new forms etc. Absolutely anything that could have been in the pipeline since Gen 2 but has never actually been used yet as they're not sure how to implement it for whatever reason, or have just never gone back to it to fine tune it. We'll pretty much never find out the majority of this, but unused ideas from the past could always make it into a future game and a lot of the 'development' for it would have already been quite far along.
I'm more than at peace with Game Freak and everyone involved in the Pokémon games because as a fan for 20 years or so, it hasn't changed much and I quite like that personally. I would hate for something drastic to come in that changes core mechanics in an instant, so the incremental approach suits me as a customer more. You have to understand that most of us posting on forums and social media, the 'hardcore' fans, the competitive players and so forth are in the strict minority when it comes to the target audience for these games. We probably make up less than 20% in the grand scheme of things despite what we want to believe. Game Freak is kind of forced to prioritise appealing to the younger market and will have strict deadlines that are timed to match other Pokémon media such as the TCG, anime, and probably more now. Along with this, it's obvious that investors and everyone involved know that these games are guaranteed to sell well in excess of 10million copies, probably in excess of 20million when you add the third version or sequels, so the yearly release when possible is just such a massive source of income that it has to happen.
I can understand a lot of people's complaints and I do think a lot of things should have evolved and been included in these games specifically, or even earlier for some things like Pokémon cries and move animations (a lot are similar to what they were in Gen 3 for example), but it hasn't happened and we can only hope that they improve more than what we've seen to this point, or at least in future titles. A big thing I always look to though is that most games that come out every year don't have the same scope as Pokémon and will likely have much larger teams working on them as well. If we really wanted this fully fleshed out Pokémon title, with MMO features, amazing graphics and the like, it would probably be a game that requires 4-5 years of development and probably means no core RPG coming out in that entire time. Some fans might be accepting of no real Pokémon game for four years if it meant we get an absolute baller of a game worth the wait, but I'd wager most fans wouldn't want to wait this long and unfortunately, I don't think Game Freak would ever be given this amount of time. The games are too big of a cash cow and it will continue to be milked on a 12-18 month basis with quite minor upgrades each time. I can definitely understand people's frustration with this as some will definitely feel they're being taken advantage of, but then perhaps this just isn't the franchise for them anymore?