The Calamities were engineered with the idea of rebuilding being possible. The Ascians do not induce them and then grind the survivors to paste, because they want the world to recover (so that another Rejoining can occur).

We have no idea if the Unsundered could have recovered, because they weren’t given that option. It’s well and good to speak of resilience, but no one was trying to wipe the Sundered out completely either.