If you read the interviews you'd knew that it clearly wasn't fine for GF but rather something they felt like they needed to do sooner or later. And that's true. It's an unsustainable growth that couldn't go on indefinitely. Even they striped everything away to the point that the game would only be a pure battle simulator with nothing at all attached to it, I'm pretty sure that this would only go on for one or two more Gens before collapsing and that's speaking from a balancing point of view alone.
It's not even close to unsustainable growth.
Considering USUM has 807 fully rigged, animated and with complete models that are ready in their current state for implementation in future titles, They need a texture rework that's about it.
Here's Mewtwos texture from the 3ds
Here's the texture from PoGo (Armored mewtwo skin)
Here's the texture he has in SwSH
The models being used are 3ds models for the VAST VAST majority. Some edits like removing unused and unseen vertices. The textures used are higher resolution, GF most likely already has higher resolution ones that exist anyway (Pokemon Go) but if not you can upscale and paint over the upscale it's not a big task by any standard.
So that brings us to animation, Considering Let's go animations work here and they are just ports of the 3DS animations for battle and pokemon amie animations and
walking animations and running animations also existed for all 807 pokemon in SuMo what animations are New here? What unsustainable work would be required?
We have walking and running for the overworld encounters, We have idle and attack animations for the battles and we have the amie animations for camping.
There's 94 new pokemon in Galar including regional forms.
Gen 1 has 151 Pokémon.
Gen 2 has 100 Pokémon.
Gen 3 has 135 Pokémon.
Gen 4 has 107 Pokémon.
Gen 5 has 156 Pokémon.
Gen 6 has 72 Pokémon.
Gen 7 has 85 Pokémon.
That's perfectly inline with the sustainable growth seen prior for years and years.
When you're not actually doing anything to the prior stuff other than porting it forward and some shaders for your new lighting engine the growth isn't unsustainable that was the point of making all these HD compatible models for a 3DS game.