Fiat currency will never disappear because governments are too happy with it to give up their monopoly without a fight, and even if cryptocurrencies are somehow able to become the most used currencies around the world you can be sure that banks, governments and even private businesses will release their own coins and eventually they will begin to gain acceptance once again among the population because they do not really care about the principles of freedom in which cryptocurrencies are based, they only care about how easy is to use and spend a particular currency.