Whatever is commonly saved, becomes a unit of mutually understood account, and for reasons of convenience tends to become the media of exchange as well.
If you and I know each other and we both SAVE the same thing, I can give you gold (or bitcoin or whatever) rather than giving you dollars which you then have to convert.
Thus the key to replacing the dollar is NOT to replace its currency function - it is to replace its saving function.
This is why precious metal will be recognized as the money and Bitcoin will not.