I believe Bitcoin will not totally replace fiat currency. Bitcoin will be added as a currency that will be an alternative to those who don't like to use fiat currency. Don't expect that the government will give in and let cryptocurrency replace fiat currency.
Fiat will remain and bitcoin will still be staying because of us who are still using bitcoin until today and still believing that bitcoin will recover again soon. I know that there are some governments in some other countries that don't want bitcoin or cryptocurrencies to include with their currencies because they can't get tax from there, but there are also some governments that don't don't care with it because most of their communities are already paying tax and they are already getting millions with that and bitcoin/altcoins is not a hindrance for them.