Sometimes people forget that there are still active online communities for single player games that have been out for years, and people are still passionate about them. You only have to look at Looking Glass and Bungie games. Why do people think that a game needs multiplayer to be a success or to enjoy longevity? It's a ridiculous argument.
Besides, does it even matter if you only play a game once, or if it doesn't have a long lived online following? Of course for most SP games, even great ones, eventually the fan sites close and the forums dry up, because after a while there's not much to talk about, but does it mean the game wasn't good, wasn't worthwhile?! The very nature of single player games is different, and the reasons people enjoy them are different...
I have so much to say on this matter, but I really don't want to ramble on and end up with a post no one will read (plus I'm at work)