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.
Crypto investments are one of the fastest growing investments in the world but I don't think crypto is ever going to replace fiat simply because of the huge acceptance of fiat. It is backed by the government therefore fiat are accepted everywhere in the world. I don't think Bitcoin would ever be able to become that much popular.
Another advantages of fiat is its offline use. Fiat transactions don't require the use of Internet whereas Internet money is nothing without Internet.