Not true. There will always be a great number of prople on the earth willing to pay a good price for gold. The gold price has been high throughout all falls of all empires and all catastrophies affecting great nations throughout history. You overestimate the importance of America to the survival of the world. Even Iraq survived our sanctions with a semblance of an economy. While an American catastrophy would certainly mean economic depression throughout the world, it would not mean the utter collapse of all economies, especially if the people of the American states had a dependable store of value that gold has always been.
In that event, the standard unit of currency will be a copper-jacketed lead slug.
No, it would be gold, as it has always been and what the founders intended. Even in the worst wars, only a small percentage of the population die by violence.