I think you're forgetting how flat many of the characters are in BG 2 with maybe 2 -5 lines of voiced dialogue each and how much work Larian has left to do. BG 2 could only rope you in to caring about a companion if they had their own quest or lots of backstory(all delivered through text.) Now every companion has to be like that and it means we get less companions but more complex backstories for them. I believe less is more and wanting to have support for a larger party is fine but it is not a priority for them right now and that is reasonable.
BTW Dragon Age Origins set the precedent of modern 3d RPGs using 4 party members (devs called it a spiritual successor to BG) and it was quite effective and fun.
I disagree with the depth of the BG2 characters -- some were a deep as saucer and some were complex. Minsc was a great, shallow character and Jaheria was deep. I rarely got to the end of Jaheria romance there were so many interactions. And BG2 was loooong. I would take me 80 -- 100 hours to finish a completionist run of BG2 / TOB.
Personally I don't think it is "reasonable"* I think they took up the mantle of the most replayed game in RPG history and need to be held to that standard. I'd be interested in a comparison of lines of text for Astarian and Jaheria to see if the BG3 characters are in fact deeper.
This is just what happens when you take up a beloved franchise -- you get many, many more sales but you are held to a higher standard. Rogue one is a entirely decent sci flick -- but that's not the standard to which it was held. Again, such pressures come with the territory.
*reasonable wouldn't have been my choice of word, only using because it you did
