Yes, but why are we even having this discussion? Nobody's asking for Larian to "shift to a 6-person party." That's a completely false claim. What we're asking for is the OPTION to have a 6-person party, which is fundamentally a different thing. It is an option, and people can CHOOSE whether they want to use that option or not.
When games come with multiple difficulty settings such as story, easy, normal, hard, brutal, do people actually believe the game is optimized and balanced for each of those settings? In every single such instance, the game is only balanced for the normal setting, and it is understood by EVERYONE that if you the player opts to select one of the other difficulty settings, you are essentially playing an "unbalanced" game. This is the very definition and essence of something being OPTIONAL.
It very much matters that game difficulty is made roughly independent of party size. Otherwise, if playing with 6 characters is strictly easier than with 4 characters, then for example the final Tactician Mode will be a joke with 6 players. In such a case, players wouldn't be able to enjoy playing with 6 characters (for the character interactions and party flexibility) AND play a very challenging game. Unless Larian balances Tactician Mode expecting that players will play with a 6-person party, which...I don't really like either for similar reasons.
I don't agree that games are "only balanced for the normal setting." Developers definitely tweak the properties of each difficulty mode to achieve some desired gameplay experience. I can point to DOS2's Tactician Mode where Larian DID balance and optimize the game (particularly the AI) for that mode. Sure, not as much work is put into fine-tuning additional difficulty modes as is put into tuning the normal difficulty, but not as much work does not equal zero work. What is your definition of "balanced"?