The European Parliaments Committee on Economic and Monetary Affairs (ECON) has come up with a report on cryptocurrency entitled Virtual currencies and central banks monetary policy: challenges ahead. The finding of reports shows that cryptocurrency will not replace fiat currency, even in the long term.
https://bit.ly/2NeLgLrStates will never agree that a decentralized crypto currency will replace their national money. Even their own centralized crypto currency is also unlikely to replace fiat, most likely all these types of crypto currency will only go along with national paper money. Decentralized crypto currency can not be controlled by the state and is very volatile in price. These two factors will never allow any state to agree to replace this crypto currency with its own money. Apart from these, there are other factors of economic and political plan as to why this can not happen.