Fiat currencies are used widely by almost all people in this global world. Cryptocurrencies can never replace fiat currencies inspite of the fame crypto has gotten some years now.
It is evident that even the toughest crypto fans cannot live without traditional money. They regularly use fiat to buy Bitcoins and altcoins as well. They sell and buy things for their national currency.
Wait a minute and think are you feeling to use regular currency any more I am sure your answer will be no and if your answer is yes then sorry to say that you really need to wide up your information and your knowledge related to bitcoin, as I personally was thinking like this but after starting to use bitcoin I dont even remember if I used fiat to buy something since last few years.
I admit we have been able to cover some of our life's needs with bitcoin, say like paying bills because we can pay for using crypto if the service is available, because I also use such services to pay bills using crypto. But not all of us can pay using crypto because there are still many marketplaces that do not provide payment using crypto. So even though fiat had replaced models or images in their national currencies, but I think crypto cannot replace fiat for many reasons.