Fiat will be replaced by crypto currency but i dont think it will be bitcoin. Government want to control everything and even fiat money , bitcoin cant control by the governement so they will create thier own and will use blockchain technology to make a new way of payment before fiat currency fall.
I don't think it will be bitcoin either because it will be impossible to control since it's decentralized, government will be able to control coins that are centralized only e.g XRP