Honestly... not to discredit everything else but I feel like the Pokémon spirit is all in here really.
There's something to be said about being able to stand self contained on its own rather than being just another gear you keep throwing into an increasingly messy pile because there's financial incentive to do so. Not to say there is no benefit to expanding but it's also undeniable how it dissipates some of the original essence.

Imagine if after the Mona Lisa was made other artists stretched the canvas and kept painting a bunch of other people around her. Sure those could still be beautifully crafted on their own right(though the more you add the more chances of missing the mark) but they would stain the Mona Lisa as a concept regardless. You could argue that objectively she's all there still so more content just means more things to enjoy but do you really want to have to bend backwards, squint and put your hands over your eyes to try to cover up and ignore all the added new things you dislike as to not ruin the classic experience?

Just my perspective, hope it doesn't upset anyone.