I understand there are arguments for a 4 man party max, having to do with encounter length and people apparently having trouble keeping up with managing 6 characters and their abilities. At least some such arguments were being made when Pillars of Eternity wanted to cut down the party from 1 to 2. Also there is and argument for wanting to limit the tools a party has at their disposal to encourage creative solutions to problems and Larian seem to like that kind of thing. But the truth is this will lead to pretty much every party being the same utility based one with 1 healer 1 rogue and the other 2 switching depending on the PC class.
6 characters wont take away the utility party problem entirely. Even with Baldur's Gate 1&2 There is always high pressure to have at least the healer and rogue. But atleast 6 party members leaves some slack there to play with.
Now BG3 is obviously turn based which makes the encounter length problem more prominent than in the real time with pause titles. But I would rather leave this up to the player and perhaps inform the player that with a bigger party combats will likely take longer.
I disagree with the OP about just plonking in the option for 6 party members without any balance considerations. I'd actually prefer a tweakable difficulty with the option for the player to increase the amount of enemies in combat situations. Give the player some of the DM's balancing power so to speak. I recently played BG1 again and I was kinda sad that I could not simply increase enemy amounts and would have been forced to also give them more health. I wanted core rules but with a bit more enemies.
Ofcourse this also puts a bit more pressure for having more companions. BG1 had tons, and I'm still not sure I've ever met them all. BG1 companions were also super simple with just a few combat barks and no dialogue. So it is going to require way more resources from Larian to add a full fledged companion. Pros and cons.