The speaker says that a meteor impact, global catastrophe, or supernova could destroy the complexity humans are building.

“But this-- there is no law that says that anything that we are doing is going to culminate in, in a structure like that, right? It's always possible that whatever complexity you are doing is obliterated by the next meteor that hits Earth or that there is some global catastrophy, that some supernova- That is, uh, eradicating, uh, all our attempts to build more and more complexity.”