Could it be possible that the "US government", aka The Fed (notice I put the word government in quotes to denote sarcasm), would create its own digital currency in many ways like Bitcoin with the exception that instead of decentralized miners controlling the block chain, it was controlled centrally by "the government". They could charge a fee for all transactions, aka a tax, and they would own and distribute all newly mined coins into existence. They would have a paper trail of every digital coin in existence and where it went. Would the people accept it? Of course they wouldn't have a choice, though, would they?
Something like this will likely happen. Paper money is a bit low tech even these days so money will evolve eventually. That being said unless it is decentralized then it's still just another piece of shit they get to control and manipulate.