Yup. Exactly. But apparently logic is not @snowram's thing.
If people can CHOOSE to play with a smaller party than intended, why not also be able to choose to play with a larger party than intended? It is EXACTLY the same thing. Anyone who cannot understand and appreciate this very simple point is clearly incapable of following basic logic.
You seem to be on the left side of the Dunning Kruger curve. You don't want to understand why increasing the default maximum value would naturally make everyone pick a 6 man party, in a game balanced for 4 players.
Because they would be warned that it is not the default mode and that it will affect the balancing of the game if they do choose 6 party member mode.
Sure, I'm fine with it being hidden deep into the lobby screen or behind an obscure flag in a config file.