Crypto will not replace fiat. Fiat was used by almost all people. Some people will use cryto together with fiat because it was more advantageous and helpful. The government cant control crypto so they will force people on using fiat. And also to adopt on the growing popularity of crypto, maybe the government and banks will make their own digital currency that are centralized so they have control on it and make people use it.
Replacement of fiat into cryptocurrency is not possible and we should also look into these two differently because cryptocurrency help us out in modern way which is decentralized while fiat is a currency issued by government so they will not let it replace with cryptocurrency as they have high benefit from this way. It is good to have multiple sources of investment and payment.