I don't believe that digital currencies will replace fiat it is fact that which we have to accept it. it is true that it is very difficult to replace fiat each and every country has their own currencies and it has it own value as per their country growth it always vary so non of the country will allow to replace fiat at least for now and for next twenty five years.
It is not possible to have everybody access to internet and the android mobile to avail and its cost cannot be bared by poor people and to be honest it is not affordable to replace fiat first the world has to change completely to have access of digital world and only it is possible.
I believe that digital currencies will replace fiat but we need a long time to do this. Government of many countries are encouraging people use payment not fiat money and digital currencies will resolve this.
every country has their own currencies
They can generate digital currencies their own and can use bitcoin conversion . Like now we are use dollar to conversion between other fiats money