Pretty much this. We may have lost roughly half the national dex, but the remaining half of the national dex is still a freakishly huge amount of mons. People need to stop pretending that having 'only' 400+ Pokémon (which is still 100+ more than the Sun and Moon dex had to offer!) means that the games are lacking in content.
It’s not really more than what sun and moon had to offer because you could trade the rest in from previous games
Sure, but you also have to factor in how easily you can access the Pokemon. To access the older Pokemon in Sun and Moon you'd either need to have purchased the older games, or trade with someone who had. It's somewhat akin to pre launch DLC. And, not everyone has the older games, or is going to want to go through the effort of looking online for trades, so for those people, the Pokemon that you can transfer in are irrelevant.
Personally, I played through both of the 3DS games without ever coming across a transferred Pokemon. I just didn't feel like transferring or So, for a player like me, the 100 extra Pokemon I will encounter in the base game are more meaningful than the 400 I could trade or transfer.