1. Game Freak is no longer focusing on Pokemon's development.
2. There has been a sharp decline not only in quality, but in the amount of effort and (financial) resources put into each game.
3. Sales have actually improved, possibly as a result of the continued decline in quality but increased accessibility.

My theory:
Game Freak is going to put the Pokemon franchise into a zombified state: Putting as little effort and as few resources into the development of the games as possible, banking solely on the "guaranteed sales" from people who will buy the games regardless while abandoning all other portions of the fanbase. Once even the guaranteed sales dry up, ie. the whales get fed up with the low quality of the games and stop buying them, Game Freak and Nintendo will finally kill the franchise entirely now that Game Freak has other IPs to fall back on.

At this point, death would be preferable for the franchise. Sword and Shield will be the most accessible ie. worst games in the series and Great Sword and Great Shield will have even fewer changes than USUM. Quote me on this next year.