I don't think anyone is demanding that the game be built around solo playing, rather that it remain a possibility. The original games were in no way balanced for solo play, though maybe there's a design doc out there somewhere saying that it was. Most solo players that I've seen are exploiting the system in an intelligent way and also cheesing quite a bit through the game. I like the idea of solo play but have usually just done party based stuff, so I'd like if BG3 was like the originals in this respect. Made for a party but soloable if you know how to bend the game and use it to your advantage if that's what you choose to do. Personally, I like a big party because it brings in all those interesting character personalities, and it was hard choosing who to bring along and who to leave behind. That said, I'm not hung up on the party being smaller. It'll probably give the game a more focused feel, potentially.
