There's no reason for campaigns to end. First, the developers of this site is earning good money. Same goes with the users of this site. So, I think it's impossible for the owner and the users to abandon or close this site. However, maybe it's possible if one day the web site will crash for no reason or if it's hacked.
There is a reason for the campaigns to end. They destroy the forum's content quality. However, from a business perspective, the reasons for the campaigns to end would be reduced advertising revenue or a decrease in user participation on the site. If that day ever comes, I imagine we would see action similar to what I have proposed previously... Adding a 2 BTC donation level (perhaps contributor) and removing or severely restricting signatures from all non-donation accounts. This would boost forum advertising revenue and increase donations while providing increased income for those willing to invest in the site. Doing this before advertising revenue or site participation begins to decline threatens to harm your userbase and thus growth by removing a source of income for new user participation not to mention preemptively firing a weapon for fighting future revenue declines. I imagine at some point this site will shift gears from fostering growth to unlocking value, but I suspect that day is a long way off.