Cryptocurrency is the future of all payment method due to the decentralized nature which grants financial freedom and power to the people. However, for it to replace cash or the Fiat, it will take years of awareness creation. Up to date, some people do not have access to internet whereas others do not know how to access it.
I think this will never happen because Fiat is used by so many large countries and it has a huge impact on the economy of the country. Maybe Crypto will be applied in a certain field but will never be able to become Fiat