Okay. I guess I'm not truly even considering parties of 1-3 at all. My focus is parties 4-6, with an emphasis on 6. You can balance all encounters for anything between 4-6 party members with relative ease using XP split. A party of 4 Level 4 characters should have relatively the same trouble fighting the Spider Matriarch as a 6 member Level 2 or 3 party. Shoot! The 4 Level 4s might defeat her easier because they have higher HP each and better stats and abilities.
But, the point is that it wouldn't take as much for Larian to implement a 6 member party. They wouldn't have to rebalance the whole game.
As for 1-3 member parties, that's on the players. You want to challenge yourself and go fight everything solo, then you just have to accept the challenge, OR set the difficulty setting to Easy or whatever it will be once they actually implement difficulty settings.
Again, that should have absolutely no bearing on this discussion about increasing party size to 6.