4 party limits gameplay for those who play multiplayer with 4 players.
6 party limits no one.
4 party = current stats and so forth. No change.
6 party = option to use proper D&D stats. Change doesn't hurt or limit anyone.
So why not give those of us who want a 6 party the OPTION to have it and to have it with proper D&D stats so it isn't so easy it's boring? Why fight it if it doesn't limit or change your gameplay in any way?
Only in as much as it limits SP. At this stage of development, Early Access, where we can't play as the Origin characters, then yes? What about all the other limits that we have because it's Early Access? As I said previously, once we're out of Early Access, and can play as the Origin Characters, your party of 4 could all be Origin Characters.
Party of 6 will trivialize the content. This is a point that has been made, and refuted with "so what".
Changing the party size as an option will have no bearing on the stats of NPCs throughout the game. The only way this is true is if they make the appropriate changes in the base game, or with difficulty modes, which again, we don't have because we're still in Early Access.