This game is designed around a party of 4 players, and that is fine. To change it to 6 would require the developers to start over on a huge amount of design work.
No, it wouldn't. It would require a re-tweak of the control system (which is DIRELY needed anyway, because the current one is terrible even for four men) and a fairly simple revamp of some encounters. Which is not an issue since most encounters in the game will have to go through months of tweaking and rebalance regardless of any change.
I wish people could stop to make up shit as armchair developers to legitimate their bias.
Bg3 isn't even a budget-restrained production, so let Larian worry about how much work it would need and let's talk about the merits and demerits of the design itself.
Party control in Baldur's Gate 3 is a complete mess that begs to be addressed. SAY NO TO THE TOILET CHAIN