Will expand my thinking. You assumption is that party of 6 should be standard, because of few reasons:
* Because previous games were like that. Indeed they were, but why we should we really stick with it? Both of this games are now archaic and are long closed story. I'm fan of both previous Baldur's Gate games, but being honest here. Without sentimental aspect Baldur's Gate 1 is just good game with average story and it didn't pass the test of time. Baldur's Gate 2 looks much better in this aspect, but still it's a relic of the past. I won't go with all this seems like Divinity 3/ Divinity: Baldur's Gate talk- not only, because it isn't related to the topic at all, but also if someone remembers how many placeholders from DoS1 were in the EA for DoS2 then there is just no point to speak about it at this stage of game/EA. So I don't really see why we should hold argument like this at all.
* Without party of 6 some players will be alienated. Yes, maybe- maybe not. However, same will happen if they will change it to party of 6. Some people would say that now it's too much of party management, too chaotic or something else. Also if they will change it once then another group will rise saying something like make it party of 8 or party of 10, because system/game/universe XYZ haves it that way. Yes your argument here is valid, but same goes for any other party size. Also like you said that party of 4 being way to go is debatable, well same goes for party of 6 being consider fix for primary gameplay mechanic- it's debatable. No matter what someone won't like the final outcome.
* Is game really made around party of 4? Yes and overall fight balance is lesser concern here. We talking about things like story, narration, dialogue system and other mayor mechanics and aspects of the game. This project is huge already which is reflected in pure numbers and statistics (at least compared to theirs previous game). In the end changing it would consume set amount of manpower to once again please just one group of people in the end.
Another things why should they stick with party of 4 is overall vision for the game. They had something in mind and it's one of the core/ground rules/aspects for game like this. It's kind of like asking devs of Pathfinder to make game around party of 4 for various valid reasons. Sometimes creators just must go with things that thought will be the best for theirs game.
Valid solution would be adding another play mode with party of 6 and maybe things like Active Pause mode in Enhanced Edition sometime after the premiere. This game really needs valid polish and improvements, so there is just no time and it's unnecessary to try please everyone with very personal taste based things now.