The world changes, so the tiresome topics change and so do the participants.
People who rose to the debate for every gay marriage thread over a period of years are tired, which is fine because the issue is tired. There are some vestiges of debate out in the midwest, but seriously, is there anyone who doesn't see either the dawn of the new day or the grim writing on the wall, depending on their point of view?
People who over invested their CFC OT persona in "gay marriage is an abomination that we just can't let happen" will leave, because the world didn't go their way. There is probably nothing to be done about that, because people who invested their persona in the other side are going to be smug about it. Those people may come back with a new persona, which may call for either a new screen name or some passage of time.
But there is always a new tiresome topic, and someone to take the unpopular side. And the world will either provide those who take the unpopular side the reward of seeing it become popular, or it won't. For what it's worth, I think a lot of the people who are "responsible for running off conservatives" consider me to be a radical, and I foresee many tiresome returns to the topics where we differ. As the conservatives drift back in they might find themselves on the same side of those topics as the people they used to dispute with. And the world will either back up me, or them, eventually.