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.)
Low amount of voice acting makes companions flat? What? (Or did I misunderstand? But the way you wrote it does make it seem like it's implied.)
And no, companions in BG2 had A LOT of interaction, both with the PC and with each other. Not all were equally good, that's true (looking at you, Nalia), but most had quite a lot of depth. And as KillerRabbit said, some were shallow - but I think there's a place for "shallow", fun characters, both in old and in new games. Perhaps you're thinking about BG1, in which, yes, companions had just a tad more depth than cardboard. Fun cardboard, but still cardboard.
Now every companion has to be like that and it means we get less companions but more complex backstories for them.
Usually I'd agree that quality>quantity, but I think Larian went too far. Yeah, the origins/companions have massive backstories and lots of content, but there's very little in terms of choice, and choice is of great importance in cRPGs. It won't do me any good that companions have so much stuff to them if there's not enough to choose from that I can have a party I actually care about. Not because of better/worse writing, but because I subjectively like some characters and dislike others.
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.
Unfortunately, it did set the precedent. I was actually very disappointed with there being only four party members in DA:O. Apart from that, it was indeed a great, fun game. And I'd even say a decent spiritual successor.
