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
Crypto and fiat working differently while some country ban the use of it because of illegal transactions but still it cannot replace the money that the government created. Both can be used to purchase and invest but only one is more convenient. It is important that you have know how crypto works so that you can survive even when the downfall comes.