Diablo 2's got a semi-vibrant modding community that has done a fair amount with the game that would not have been possible without single-player/open multiplay

Diablo 2's got a semi-vibrant modding community that has done a fair amount with the game that would not have been possible without single-player/open multiplayer. I agree with you that the vast majority of players probably will only want closed, online multiplayer but it's hard to imagine it having the same legs as D2 without.