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.
Well it is true that nothing is permanent but we all know that it is not that easy to replace something that is in there for a long time of period, fiats are obviously one of them. You can't take out fiats that easily since there are a lot of people that are used in using them. That day might come but I don't know if I am or you are still alive at that time.