"Unbalanced difficulty and frustration will stop them. Is it so hard to understand, that there a lot of people who liked NWN1 being centered around sole PC, who only bought OS2 because they could play it with only one PC? People who like parties may want 6+ PCs, but they will still be happy with 4. People who want to play with controlling only one PC won't buy and enjoy a game where they will feel handicapped playing that way."
But again, play it solo or with party of 6. Use proper XP split and it works. You play solo and gain 4,800 XP. You are now 1 character at Level 4, going on to 5. You play party of 6 and gain 4,800 XP. Split 6 ways, you have 6 Level 2 characters. Same challenges, same difficulty because 1 Level 4 = 6 Level 2's.
So, why not allow the game to be played with up to 6 party members so if you want to solo, you can, but if you want a party of 6, you can also?
This doesn't work with DnD system. 1 level 4 =/= 6 lvl 2s. Also in the endgame you hit content designed for 6 characters at max lvl while you only have 1. Try playing Pathfinder or BG2 with a casual not crazy min/maxed character even on normal difficulty without cheese, it won't work.