If they let us have a party of 6 AND custom companions they will need to add harder modes, or veterans of DnD and min-max players will stomp the enemies in every fight.
That's kind of the case now. Browse the forums and you'll see it's full of people who are breezing through combats just like there are people who are taking several reloads to succeed in the same ones. Your argument is basically that people who are good at the game will have an easy time, which is already the case with a 4 person party.