Why has it taken this long for Pokemon fans to finally start complaining? The flaws in Sword/Shield are flaws that have been apparent for several generations now.

1) Pokemon games have always been rushed so that they can maximise sales. Back then they used to add some of the cut content back into the game and rerelease it as a "third version", forcing fans to pay full price for a "new" game. Now they have stopped that practice but instead they just move onto the next game instead of fixing their games. Modern games can now release updates to games post-release, so there's no reason why they shouldn't been doing this.

2) Game Freak still operate as a medium-sized game dev studio when Pokemon is the highest grossing media franchise in the world. With 2D games, a lot is left to the imagination so you can generally get away with putting in less effort. But with 3D games you need to put a lot more effort into the models, animations, textures, etc. for it to look as presentable. With a game like Pokemon where there are so many unique monsters, it will require a lot of additional staff hours to accomplish this. But Pokemon certainly generates enough money to cover that cost. Even if Game Freak don't want to hire more people, they can always have other partner studios working with them (e.g. Monolith helped Nintendo to make Breath of the Wild)

3) Other video game franchises have evolved as technology improved. Other flagship Nintendo titles like Mario and Zelda took full advantage of the console's capability. Meanwhile Pokemon was stuck to 2D grid-based movement until 2013. Admittedly Sword/Shield does seem to be doing a bit more to make the world feel like an actual place where Pokemon and humans coexist, but it still has a long, long way to go.