I don't see fiat currencies will disappear specially in the First World Countries. Nobody took a power from governments to make their own money since every federal and state government has a lot of government or city jobs. They will pay salaries in fiat currencies to them, private sector might choose different currencies to compensate their employees. I think crypto and fiat will live together and may be compete. In the Third World Countries crypto might take place of fiat, because of the fast inflation rate and big corruption of a financial services with government.