The cryptocurrency couldn't really replace fiat currency because fiat was supported by the government and fiat money started to dominate since the 20th century, that is why it is being used globally. We also do not need for cryptocurrency to replace fiat because cryptocurrency should always be decentralized.
It is not that it is supported but it is the currency being controlled by them. They would never let crypto currency replace fiats since we all know that cryptos are decentralized, as no one would control it, whales would be able to manipulate it. I think crypto currency would go side by side with fiat, as we can use fiat every day, that would be the same as crypto currencies.
I agree on that crypto will not replace fiat, because fiat is really controlled by the government and it is really more trustable than crypto this time. Because there are a lot of tricks in crypto that can be easily become scam and everyone may become a victim on it and lost its trust in crypto while in fiat we can all see that it is very much more legal than everything. Anyway we can still both used crypto and fiat and theres no harm on doing such thing.