I think cryptocurrencies are hard to replace fiat money or it will take a lot of time. Because cryptocurrencies are decentralized, decentralized, unchecked - something governments don't like. Money is the power and power of each nation. Therefore, cryptocurrencies are always constrained and banned in many countries
Even when cryptocurrency is legalized, its use will necessarily be restricted by the government. Cryptocurrency cannot completely replace Fiat, otherwise the state will lose control over its citizens. Here, the government will most likely use the national digital currency to maintain control.