Even if the cryptocurrency are very popular, but we can not totally replace fiat currencies ,beccause cryptocurrency has limitations of places where it is to used where there is internet access , it can not ve used to places where there is no internet access ,so fiat is still can be used as medium of exchange.
Yeah you are right but I think that if the government accept cryptocurrency and legalize it they can control it and also 20se it for their transactions. In the future I think there would be no need for fiat currency. There will be only one currency in the world and that will be crypto currency and bitcoin will be the mainstream in the future in the globe.