re: #188 Belafon

How many examples do we have of this?

Plenty of examples here on earth: the Greeks and the Romans both were advanced, but imploded or were overrun by barbarians. The Mayan civilization was doomed by climate change. And the dinosaurs were wiped out by an asteroid. So there are a host of scenarios under which our civilization can be wiped off the face of the earth: internal violence, plague, an uncontrolled Chernobyl, etc.