Increasing the default party size would take (some) additional work, but allowing an optional increased party size requires negligible work. All Larian would need to do is include a checkbox in settings "Allow parties of 6" with a warning when you click it "Warning: The game is balanced with a party of 4 in mind. Having a 6-person party might not result in the intended experience." As Tuco says, BG3 will be released with different difficulty options, so those who want a challenge but also to play with a 6-person party could up the difficulty.
(Quote from an interview with Sven Vincke:) "The studio has still opted to use the common Kickstarter method of going to early access before a main release. “It was never in doubt,” Vincke says. “We've seen what early access did for Divinity: Original Sin 1 and Original Sin 2, and so we know how much a community can add to the game. That is the case already for Baldur’s Gate 3, so I think it was the right decision.”
I've seen all the companion questlines and right now, I'm mostly interested in testing different team combos. If they want to use EA for player/community feedback, then let us do that. Let us actually test things without having to use these clunky workarounds. Let us test 4 man parties, 6 man parties, 3 PCs and 1 NPC parties, 2 PCs & 2 NPC (1 romance for each, why not?)... see what works for us, what is ridiculously OP, what is plain unfun.