> A silver coin is worth its weight in silver

Which may be essentially nothing.

Native Americans (e.g., the Aztecs) liked gold because it was pretty, but did not use it as any kind of medium of exchange. By all accounts they were quite puzzled by the obsession the Spaniards had for it.

Silver and gold are useful as money because they are scarce, durable, and recognizable.

I think you'd find that producing convincing imitation U.S. bank notes or even coins is considerably more difficult than you're imagining, especially in a global collapse scenario.