It will be quite a very difficult thing for crypto to replace fiat. People still trust the fiat system more than cryptocurrency.
That is for now. It will happen eventually. We just cannot see it yet. Maybe we wont see it either but I have hopes my son will.
First of all, Crypto is never going to replace fiat, ever.
Second, hypothetically if this happens and Govts. are using crypto as fiat currency, then what is the difference between fiat and crypto. What would happpen with decentralization then?
They will have their own crypto. If it is digital then it is faster. There will be no more papers to bring everywhere you will go. Banks wont have to count the money they took from you. That is if they still exist.
If it is bitcoin which is going to be used then I doubt there will still be a government. If there is, then they should count on the people to pay them since everything will have privacy from then on.
One reason why they ban it also in other countries. It will stop the large amount they are taking from the people's tax.