What happens if I leave my society, or if it dies?

If you leave your society, it carries on without you; the other Cultivators living there keep building and running it as usual.

Societies don't really "die" on their own. A society lives on as long as people are part of it, and it can always welcome new members to keep growing. In some cases, if a society can no longer progress or is left without any active players, we may remove it to keep Avesta healthy, but a community with active players will keep going for as long as its players do.