There is a high possibility that US dollar fiat become crypto currency, just like what Japan is preparing to do. Japan is considering making their own digital currency called J-coin. Much like what will Japan intended to do to their Yen with J-coin to be a parallel currency. So I think the US digital dollar will be a parallel currency to fiat dollar in the near future. If you can't beat them, join them.
It will always remain parallel weather btc or crypto are still a huge threat to basic functioning of economy , we saw in case of bitfinex not once but twice circulating a online currency can pose a huge threat not everyone is educated to be able to tackle potential risks , not everyone has internet or the minimum technology which will be essential for it's functioning.
There has also been a battle between crypto and the US dollar. Crypto is increasing day by day. Pose a huge challenge to the world's money managers.
Not really i don't see much difference in the world people still and should prefer to use local currency btc is meant for making profits.