Too be honest I don't see good reasons to change it now. From most players perspective party of 6 is old fashioned and mostly just matter of personal taste. From sales perspective Larian way of doing things works and most people agree with it. Finally the most important from game design perspective everything is written, design (gameplay, multiplayer and so on), made and balance around party of 4. So chaining it now would created unnecessary chaos to please just the one group of people.
So making party of 6 a basic feature of the game? Big no. Making engine handle party of 6 so mods will allow it without any problems? Yes, go with it.
From a sales perspective, OK I mean they already got my 70 some odd bucks. I'm sure they could cash out now, not change anything substantial out of EA feedback, "stay true to their vision" or "stick to their guns" or insert whatever euphemism we like for essentially ignoring the feedback, even after consideration. But it'd also make me a lot less inclined to buy an expansion or sequel if that was their approach.
Party of 6 isn't a complete deal breaker for me, though I really do think its better on the merits and not just purely based on my nostalgia for the archaic. 4 does not feel Epic enough for me, but perhaps if they listen to feedback in other substantive areas my disappointment about that can be allayed. I actually think it will be better for the points like narration, story, dialogue system and every other mechanic I can think of to have 6. Most issues I have with micromanagement currently have way more to do with UI organization and pathing fails, than the number of PCs I'm controlling in the party. Everyone is already following me to camp anyway, so I basically already do have a party of 6, at least for many of the narrative components, I just don't get to have them along for the ride when actually playing. It may be a matter of taste, but I'd much rather have 5 companions than 3, since I think it would be more engaging all around.
I bought this thing mainly because I wanted to support the franchise and have high hopes for BGIII. I fully expected the EA to be half baked, and this still feels very much like an incomplete game atm. I've enjoyed it, but its also missing many features that I would have expected and the lack of the 5th and 6th party slot is not the clunkiest of clunky things going on right now.
They need to work on stuff like camera control and party movement pathing, inventory and spell management, hotbars and general UI stuff too. Which all seems way more important to me than scaling the difficulty levels around a particular party size or balancing combats etc just yet. Balancing is a long haul. How much balancing are they going to be able do anyway if we are only testing a 3rd of the game in EA? I'd prefer 6, maybe I'm old fashioned though I don't know. I think this is low hanging fruit, and would likely please more people than it would annoy, but that's just my anecdotal sense of things. For everyone motivated enough to actually post on forums, leave a review, or join a discord to yammer away about stuff there are probably a couple dozen players who just go with what's given and wouldn't be chiming in. I don't have a sense of what most people want, but I know what matters to me. I think they'd be unlikely to get another 70 bucks out of me at this point. Especially if its just like thanks for the feedback and for helping us to find and crush all these bugs, but we're going to hard pass on making any major changes due to already sunk costs. If its take 'it or leave it', I'm already taken I guess, but it leaves a bit of a bitter taste, since I really do have high hopes for this one. Plus it just seems like not particularly complicated to pull off. It not like they have to rebuild it from the ground up to make this happen. If its already in the MP game it should be there in SP too.