So don't play in a 6 man party. What he's advocating for is the ability to do so. I think you're both shooting down the idea in spite of the fact that the game will be balanced around 4 people anyway so your experience wont change
If you balance the game around 4 characters but allow a party of 6, everyone will have 6 characters in their party anyway, and this would make the game too easy. If you want to go that route, it makes more sense allowing this option with a mod instead.
If the combats would scale their difficulty automatically based on party size, I think the option of 6 is fine. That being said, it's a tall order for Larian, which makes me doubt they will take this route:
Balancing this system would be very work intensive
Companions are very well fleshed out. Having to add more is once again very work intensive. And allowing hirelings just to fill out a party of 6 seems... odd.
EDIT If people feel combat is too hard, that is a different problem in my opinion. That does not necessarily need to be solved by adding more party members. It could instead be solved by making the encounters easier.