Not everyone who left Digg went to reddit, and not everyone who left Myspace went to Facebook. "Replacing" reddit should never be the goal, it should be "be better than reddit".
If this is ever to go mainstream, what we should be concerned about is making good, high quality original content. If people see us having fun and being nice here, they'll want to join in too.
Great point about the high-quality original content. I remember before Reddit was popular, that’s where much of the original content was generated, and it would eventually be reposted on Digg. Reddit had the reputation of being tomorrow’s Digg homepage today.