Monsters should deal more damage, or have more HP, or just be more of them...
All the re-tweaking and testing, and further tweaking and testing of the already existing content (also act 2-3-...) for 3, 5, 6 party members would be a lot of time. And if they implement it, then people would start complaining about how they implemented it, and why it should be done differently...
As I keep saying, they can simply NOT increase/scale/tweak anything.
Just a toggle to increase party size with the following note next to it: Combat encounters are scaled and balanced for a party of four. In choosing to increase your party size, you should understand that encounters may become considerably easier.
