I've even seen a number of subreddits with no moderators/inactive moderators which were pretty good, even with a few thousand members.
I suppose might be that the forums you're discussing may be one of the ones created with the express selling point of having low/no moderation, while these other places where created as a place to have discussions first and foremost.
Emphasis on "the old". Usenet in the early to mid 90s was pretty great too, but again, emphasis on "the old". Niches that haven't been discovered by many people yet will likely always function well.