I think bitcoin will not replace dollar.
However, fiat currency still required by state to build the economy of the state itself,
fiat currency is fully managed by the government,
with the value determined by the quality of the economy of a country..

But about bitcoin, we know that is fully managed by the 'system',
and the price is very volatile, certainly too risky if a country uses bitcoin as state assets and transactions daily in a country.
moreover, until now there is no legal entity that protects the legality of bitcoin..
But that's just my opinion