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
eh, I liked the Isshu Pokemon. 150 isn't really enough, I'll agree with that though.
I don't mind having older Pokemon reappear to some extent, although having an entirely new Pokedex certainly does have a high novelty factor. I just wish it were the minority that were taken from previous gens with the focus on new Pokemon. I think the Alola/Galar forms are a good way to help with that actually, since they're kind of reusing old designs and giving fans some nostalgia, but it's still partially a new Pokemon and gives the new region some uniqueness that it wouldn't have if it was 100% the same as the previous time that Pokemon had appeared.
TBH I'd be kind of annoyed if this game wasn't 100% the Sinnoh regional pokedex though. I don't really like them trampling on the established lore of the Pokemon world. Those were the Pokemon in Sinnoh last time so it should be the same ones this time too. Sure they could use the time difference to try and explain some different ones, but I don't think that works so well when you're going back in time, that would basically be saying some Pokemon that used to be in Sinnoh went extinct over the last couple hundred years.