There is no realistic scenario I can think of that would make SE close the Math site, it's pretty successful. But even if for some reason SE would shut down, all the content is licensed under a Creative Commons license that allows you to reuse it somewhere else (as long as you provide proper attribution).
SE provides a database dump of all graduated sites every few months, you can use that to set up a copy of this site if you like. For every site that was shut down (only beta sites have been shut down, no graduated sites), SE provided a final data dump with all the site content.