in my opinion to replace the fiat currency is not possible because the legality of the crypto currency is not recognized by all countries in this world and they still believe in the fiat currency of course
One day things will change and crypto will take over the fiat currency. Nothing is permanent and forever in this world. Sooner or later, this transformation of currency will happen and then new generation will only use crypto currencies and Fiat will be just a history.
although that will happen, I think, there are still so many shortcomings that cryptocurrency has, because the price is very unstable. when bitcoin replaces money, of course the price of goods will rise and fall rapidly, if you use cryptocurrency as a standard and substitute for fiat.