Personally I'd trust that if Larian want to make & balance the game around a 4 person group, then that is the optimal way to go. In fact I'd be concerned that if enough people keep clamouring for a 6 man group then they will end up using time & resources to cater to them, to the detriment of other aspects of the overall experience.
Sounds like the marrying of a baseless assumption and a pointless concern. Tweaking the UI to adjust for six characters wouldn't be that much work (especially if they also take the chances to improve party control, which is sorely needed regardless of party expansion) and encounters are still in the middle of balancing/tweaking, so better address the idea of a bigger party now rather than later down in production.
It's not just tweaking the UI though is it, entire maps are designed around encounters and battles. They would have to add quite a few more companions into the game too, otherwise you'd just end up taking the same ones every time. Which means designing, writing, coding, voice acting. I think there were 16 potential companions in BG 2 so if you're looking for this level of variability then that means tripling the amount of work they've done so far in terms of the above.
Just for the record, if this is something that Larian already have in the pipeline, then great, I'm all for it. As long as they can achieve it while still having enough time and resources to do everything else properly.