but I'm almost certain there isn't enough extra content to keep someone playing long after all the main quests (+faction quests) are completed.
I've noticed that tons of people are creating several characters to do multiple play-throughs
but other than that, I don't see how someone could truly play this game nearly forever.
edit: obviously I don't mean forever quite so literally.
still playing regularly after a year or two after finishing the main quests or
playing once in a while for longer than three years would be "playing long after all the main quests are completed".