People use US currency in some other countries now, independently of the US government. Nothing can stop them, and if the US as an entity went away, nothing wou

People use US currency in some other countries now, independently of the US government. Nothing can stop them, and if the US as an entity went away, nothing would continue to not stop them, until the currency wore out, I suppose. Also, I believe I've read that the US does not declare old money invalid like a lot of countries do. It might be a collector's item worth more than face value but in theory you can still spend it.