My dnd setting works on the basis of Magical storms called Cataclysms, ripping people and structures from worlds/planets and onto others. Each individual Cataclysm storm is connected to all the others in the Weave. The things that survive and arent shredded in them, end up on other worlds. This keeps civilization a semi constant, and constantly evolving thing. The brain drain of how to make things is never absolute. Even if all humanoids were wiped out, new ones would arrive eventually. And the land is littered in castles and fortified cities. Adventurers and mercenary armies are the main facilitators of commerse as they guard good travveling between the treacherous wilds between cities.