If this thing was happened i dont know what about lower level of society such as tribes or rural people they can't use currencies and they could'nt be easy to access in currencies but as i know cryptocurrencies is only alternative way for paying or banking.
I can't imagine it as I know it will not happen.
Fiat = government, you can't take that out from the equation as that is the norms of the world.
Bitcoin was invented not to replace fiat, but to be considered as an alternative, it's just that people are more investing on bitcoin than using it as a payment system that's why we thought it will replace or disrupt the current norms, I'm telling you, it's not gonna happen.
Yes, government need fiat currency to run the system of the economy but it will be interested to see how the countries like El Salvador and others run in the long run once they have made bitcoin legal tender.
We can only imagine world without fiat but i don't think it is possible in the short or medium term.