I think that unfortunately there's a massive enough user base that, as long as there's a steady flow of any content of some kind, coupled with the design changes to the site (and the design of the potentially soon to be mandatory for mobile viewing app), that enough people will stick around to keep it viable. Moderators may leave, but there will always be someone, especially for the big subs, that has been rubbing their hands together waiting for an opportunity like this to take over.
I don't believe they'll last another 18 years, that's for sure, but I don't think things will crash and burn as much or as quickly as people might like.