I play most Pokemon games, and while I don't know exact numbers, it's usually the case that around 300 ~ 400 Pokemon can be obtained in a single game. The difference is that in the past there were ways to obtain them all in a single game. But has that always been the case? I remember when Ruby and Sapphire came out you could no longer trade with RBY and GSS.
Could GF be planning on releasing another main installment on the Switch which will be compatible with this one, and slowly all of the Pokemon will be obtainable. This would be a good reason to explain why not all of the most popular Pokemon are in this one game, as they want to distribute them evenly.
But overall, the disappearance of the National Dex is disappointing, but not unexpected, and 400 Pokemon is a solid amount for being within a single game. I doubt anyone truly expected more than this. But at the same time, those that are claiming to prefer having a lower amount are also full of themselves.
If you were expecting 600+ Pokemon, that would probably be a first. I'm more concerned with the MC of this game, and whether it's going to be good or not.