I actually disagree with this. I'd rather they have less Pokemon in future games rather than more.
It really annoys me how many Pokemon from previous gens they put in the new ones. The different regions in Pokemon don't feel that different if they all share the same Pokemon. I really loved gen 5 and Isshu for that reason. It only had new Pokemon, none of the old ones at all, which really made it feel new, unique and interesting. Unfortunately that doesn't seem like something that'll ever happen again, especially not with the recent gen 1 nostalgia drive Gamefreak have been on. But I'd still love more Pokemon that can only be found in a specific region, nowhere else, that makes those Pokemon, the region they're in and the game more interesting IMO.
I have mixed feelings about it
Half of Unova Pokemons are very ugly and copycat of the concepts of the original 150 Kanto Pokemons. GF proved they can't design 150 well looking monsters each gen, so they are focusing in making 60-70 better designed Pokemons and fulfilling the Dex with older Pokemons that's the best approach for the main story. It worked pretty well in both Kalos and Alola. Plus with that you can have some 200-250 Pokemons appearing before the League it means variety, BW suffers from repetitive encounters and battles because the smaller DeX, BW 2 corrected that
Post-game is another thing entirely. Removing Pokemons is essentially removing content from competitive, it's true many Pokemons aren't ever appear on VGC because they are weak, but that are a crappy of useful Pokemons (and moves) that were removed from competitive and that sucks hard
That's said I don't think Legends will be a mainline pokemon, VGC will still be based on Sword and Shield, the Dex and Gameplay won't affect that. I'd better see them choosing 100-150 Pokemins and giving them good animations and assets than having a gargantuan DEX full of ugly animations