Fiat currencies are not that easy to replace even if the cryptocurrencies are already in its prime use or trend. There's a lot of things that need to be addressed first before fiat can be replaced by cryptocurrencies.
These all things related to fiat currency will also be covered by fiat currency. When the use of crypto prevails in the world it will not only take the place of paper currency but also fiat currency. We should wait for the time when we see crypto to be our legal and regular currency. I know that it will take long time but eventually it will happen one day.