I don't think the world's going to run crypoturrency, leaving behind fiat money. Although the blockchain has already been adopted by several companies and countries, it does not mean that they will also accept cryptocurrency. Since we know there are strong swings in cryptocurrencies that are not appropriate for use in payment instruments. And also most governments do not accept cryptography as a means of payment they only use cryptography as an investment tool. And some consumers of cryptocurrencies also tend to use fiat money for payment devices because it's more convenient. They only use crypto for commercial and investment purposes.