I still am not seeing a valid reason to not have a party of 6.
What I see is the game would work best WITH a party of 6.
1. Monsters could have actual 5e stats. 2. Story triggered for all origin characters so you don't have to change them out. 3. Able to trigger origin storylines even with a 4 player multiplayer session. 4. Able to strategically design your party more effectively.
The benefits of 6 way outweigh any negatives.
And that's my point. All the arguments against 6 are not big development issues.
And don't forget, they plan on implementing Difficulty settings, so that blows the whole balance the game issue out the water even more.