Actually, even with multiplayer angle, I'm not sure why it'd be unreasonable for 6 players to be allowed? Would it require that much resources to update the menus and code to allow for players 5 and 6…?
Maybe some netcode limitation. But I doubt. In fact I think some old mod already allowed multiplayer beyond 4 people for DOS 2? But I'm not sure, so don't quote me on that.
What's really relevant, anyway, is that even if that was INDEED a limitation, there would be absolutely no need to break it. Just because the theoretical ceiling for the party members would be raised to six, no one would be forced to play with six players or even less to FILL the party even in single player.
BG1 and 2, as IWD 1 and 2, had a party of six and still a lot of people enjoyed doing even solo runs. It's the magic of scalable systems, you know.