It is not realistic that cryptocurrencies will replace fiat currencies, at least in the nearest future. Nowadays most people around the world are using the fiat money and also banks support mainly fiat and fighting cryptocurrencies. So it will take many years at least until crypto can replacing fiat.
It cannot be seen right now but we have to admin that it has a chance in the future, so we should not close the door for this as it has the chance to replace but for sure not that soon it may take some decades before it will happen.