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.
Party control in Baldur's Gate 3 is a complete mess that begs to be addressed. SAY NO TO THE TOILET CHAIN
