Not sure if this has been mentioned already (there are a lot of pages to this discussion), if so I apologize.
Let me start off by saying that I would like at LEAST a 5 party member band, but 6 would be phenomenal. That said. Anyone who has DM'd their own PnP game know that the party size influences A LOT of what happens in the game. Everything from encounters to loot to gold gained for those encounters. If an encounter is easy, you don't want to reward the group for just walking through your campaign. The more players you have, the more modifications you need in an encounter to balance it out. If that encounter is overcome, then the more experience, loot and gold that is gained from said encounter.
An example of this would be the owlbear. Right now, that is a challenging encounter. Add 2 more players, and that challenge rating drops exponentially. So in that case, the encounter would have to be modified with the owlbear doing more damage, having increased hit points, possibly even including more enemies in the encounter, just to balance it out. This would also mean more experience for your characters, which means they level up faster, which means every other encounter in the game would need to be modified so that when you are reach further in the game, you are, once again, not walking all over the enemies and getting handsomely rewarded for it.
Can this be done? Of course. But it requires planning, coding and modifications to the game which they need to be able to focus on. Larian may very well be doing just that, but it's going to take time. In essence, it's a wait and see what happens scenario. I have a feeling that even after launch there will still be more tweaking and modifications made by Larian themselves, simply due to sheer scope of what D&D is.
