Cryptocurrency can work side by side with fiat currency but can't replace it. And their are many reason behind this.
 Cryptocurrency is not legalized everywhere. In many country it is still ban. Because of this it can't call an global currency. Another reason is its volatile. Because if volatile nature its price fluctuate time to time and because of this many people lose their trust on it. Cryptocurrency is an digital currency so in order to use it Internet connection must required but their are many place present where electricity does not present and in those place fiat currency is one of the options in those place.