A couple years ago, we on the Community Team made our guidelines for graduation and site closure a bit more concrete than they had been in the past. Here's the most relevant bit:
If there's enough moderation for a public beta site to consistently remain free of spam, for flags to be cleared, and for our Be Nice policy to be upheld, your site will remain open. However, if ... we can’t find any suitable volunteers to step forward, the site gets closed.
Right now, having enough moderation is not a problem on this site. However, as some of you may have already seen, one of your current moderators would like to step down. If you're interested in being a replacement, please let me or another Community Manager know.
pops@(either stackoverflow or stackexchange).com works, as does replying to this post.