As YouTuber
Foxcade pointed out, Pokémon transitioning to a 3D game has made it a logistical nightmare. Game Freak has had to double its workforce twice to keep up with deadlines when it comes to modeling, rigging, and animating at least 6 move animations per Pokémon, on top of building a new map, programming new moves, and having to maintain a balance between catering to both casual and competitive players.
That's so much more work than just having 2D pixellated sprites with a couple animations, and then having moves animated on their own layer, a trick which Game Freak leaned on up to Gen 5. Scale that up to 1000 Pokémon.
Part of the blame is on Game Freak for setting the expectation of being able to take Pokémon with you between old and new game consoles. Eventually Pokémon Bank and Pokémon Home are going to become deprecated. Eventually Pokémon GO is going to become deprecated.
I personally believe that ripping off the bandaid and soft resetting the series every few generations like they did betwen 2nd and 3rd gen will lead to higher quality video games, a fresher competitive metagame, and probably better Pokémon designs. Then again, I don't have Pokémon Bank, and don't have Pokémon I caught 20 years ago in games I bought this decade. Maybe if they followed a formula like they did in 3rd and 4th Gen where they made smaller core games focused on the current/future Gen and let you acquire old Pokémon by connecting your game to spinoff games á la Pokémon Ranger of Pokémon Box, that could mitigate the workload?