in my opinion to replace the fiat currency is not possible because the legality of the crypto currency is not recognized by all countries in this world and they still believe in the fiat currency of course
Everything is possible in this world - try it. For crypto to replace fiat will take a good number of years for this to be achieved because cryptocurrency does not allow any central authoritative body like the governments to have absolute control over the transactions. Also it will demand every part of the world to have internet source and access in order to realize this goal of crypto hitting the mainstream adoption where fiat will be irrelevant.