Both Digital currencies and Fiat money have their own features.Using digital currency instead of fiat money is one step to become cashless but i dont think that digital currency can replace fiat money because digital currency is not accepted everywhere .It does not have any physical form so it cant be used directly in all the physical stores. Inorder to use digital currency,we firstly have to convert it into some other currency having physical form.From so many years people are using fiat money for financial transactions so it will be very difficult for digital currencies to replace this traditional system of transactions.So rather than replacing fiat money ,digital currencies will work hand in hand with fiat money.