But it’s a pretty far-fetched hypothetical scenario. You could also ask, what would happen if all of a sudden everyone in the world decided that food had no value? Gee whiz, all the farmers and restaurant owners would be screwed!
I think it's about as likely as somebody dropping something and it falling up. Keep testing the existence of gravity, it could change on you.
I'm not sure that you realize that the current monetary system is only about half a century old. Fiat currency, backed by nothing except the government, has only existed since the 1970s.