I've thought about this more and my mind went to Fallout New Vegas. It had companions with quests too but they were overall unconnected to the main plot as well. That game's storytelling worked great and it didn't really have broader, set cast either. My theory there is that New Vegas was just telling a smaller story, focused on just one single region and firmly anchored on a specific moment for our character, i.e the beginning of the game. BG3's story is supposedly about one region as well, but we have things touching on other planes and ancient conflicts, multiple gods and wannabe gods, and events going back decades and centuries before the main plot even begins. BG3's story is just so expansive that it NEEDS more of a cast to support it.
I think the game should have done one of two things. Either it commits to having a handful of unkillable essential NPCs, or it makes those essential PCs killable and if you kill them you get a non-standard game over because the story just can't work without them and you inevitably fail. I think people would be happier with the latter approach generally.
