It’s only irrelevant if you ignore that MMOs are supposed to take place in some sort of persistent world. Aren’t they? Each game has its own nebulous unspoken contract about what’s persistent and what’s for show.

That’s a bit irrelevant. Single player RPGs and even books, for that matter, work just fine if other people have completed them.

That said, I agree with the rest of your comments; MMOs are ostensibly about interactions between players, otherwise, why play online in the first place? I’ll note that such doesn’t mean direct interaction all the time, as Tobold noted recently with his direct/indirect interaction article. Still, even for those players, like me (or Saylah of Mystic Worlds, notably), who largely solo MMOs, there’s something more vital about the game world when there are other players in it, even if I don’t do much with them all the time.