simple it definitely wouldn't be possible. rather I think that fiat will probably start to get into blockchain and crypto. the use of fiat will probably become the same as a stable asset in the global crypto market. but fiat in crypto networks in the future can and is legal to be used as a means of payment in a country.
Bitcoin and ethereum have always big chances to be a medium of exchange in the coming years but it won't definitely replace fiat in all aspects. They can be used as great options for payment but they won't take place the position of fiat.
I think crypto world is still early as it is since the adoption for bitcoin and hopefully ethereum too as good currencies is still not realized. Yes they can be in the future but the exact date is still uncertain.