This isn't a tabletop game where all you have to do is recalculate some stats and do extra dice rolls. The game has literally been designed from the ground up with a 4 character party in mind. The size and shape of the map, the environments, pathways, encounters/combat zones. This would all have to be overhauled in order to optimise the experience.
This is a new game with new challenges, and all I'm hearing is people saying "but I like to do X with Y character class" well now you'll have to figure out a new way to approach it!
Well simply in your own words: The game CAN be designed from the ground up with a 6 character party in mind. The size and shape of the map, the environments, pathways, encounters/combat zones. Its still EA.
EA is way too late in the development cycle for this. We are talking about a beta build that Larian felt confident enough to open up to the public. You only go to open beta when you have everything finalized and ready for testing. Sure, they can change some things around, but the sort of change you're talking about here is to go back at least partially to the drawing board.
Of course, that's pretty much all my own opinion. I prefer smaller parties in my DnD because they're easier to manage (as a DM), and better for more in depth RPing (as a player).