I can't even conceive that China, Russia, etc. would one day demand payment for anything exclusively in bitcoins unless they're well aware that the countries they're selling to have enough bitcoins and are willing to deal in that.

Also, there's nothing stopping the US government from simply using BTC itself, or whatever cryptocurrency it sees fit, as it might any foreign currency.

I've never seen any sign that the Fed or the USG actually see cryptocurrencies as a threat.