no one can replace fiat money, crypto is just a currency made for a place to invest, crypto is not suitable to replace fiat because its value rises and falls fluctuatively
currently the use of crypto currency is only an investment tool. and has not been widely accepted as a payment instrument. so this is impossible to make crypto currency as a substitute for fiat currency.it will need long time to mass adoption.
In my own opinion it cannot replace fiat as this still used by many people although bitcoin has existing. Both has it's own features so I dont think it will replace. Many several factors to be considered and to be adopt all users.