Cryptocurrencies will not replace fiat money, nor will governments and banks allow it. In many countries where people rely more on fiat currencies, cryptocurrencies are not widely accepted globally as a form of payment. Fiat currencies will not disappear as long as governments and central banks exist.
Thats true, as of now fiat and crypto co-exist as a payment method. As a merchant, I have integrated both fiat and crypto payment gateways and I really like the significant difference between the transaction costs as I'm only paying 0.5% with capitalwallet. Cryptocurrencies have provided us the transparency and speediness that we have always longed for. So, it will be no surprise if cryptocurrencies replace fiat as a payment method in future.