Giving the option for 6 party members would be way too much work in my opinion.
How so? Please explain, because I only see extremely minimal additional work for Larian. Larian themselves have already confirmed that their UI screens are already set to accommodate up to six characters in the party. People are already cheating the game to play with parties of 6. So all they'd have to do is create the optional toggle in the difficulty settings screen to increase party size and that's it, because from a systems standpoint that fifth or sixth party member will not be any different from the fourth.