If they meant decaying in the "climate change is destroying the world" sense, then presumably the idea is that we've learned our lesson and won't destroy the next rock the way we have this one. Debatable, but at least the next rock is unlikely to have dead dinosaurs for use to dig up and burn.

If they meant decaying as in "the sun is eventually going to swallow us up", the presumably we will spread humanity across a bunch of decaying rocks, and continually migrate for the most decayed to newly formed rocks.