It is almost impossible fiat to be replaced with crypto. Once the governments see all the advantages that comes with the blockchain tech probably fiat will become crypto.
Do you mean the will replace fiat currency and come up with a centralized currency, that is a real possibility in the next 50 years but never think about that revolution to take place anytime now, majority of the government are trying to make things digitally to have a complete control over their citizens and if they come up with a centralized crypto the government will have the upper hand to monitor every transaction, but i do not think it will be a realistic thought process.