this might happen but I think it will take a long time maybe in 10-20 years. because right now the cryptocurrency mass adoption hasn't materialized there is still a lot of rejection in some countries. most governments reject crypto because the crypto's anonymous nature will tend to be used for criminal acts. and bitcoin is also decentralized and cannot be controlled by the government. so I think fiat money will remain until later.
Fiat money will be destroyed and replaced by digital money, that's what I predict for world currencies in the future. At this time Cryptocurrency has not been well received by various countries, because most people do not understand what Crypto is and how to use it. If later on Crypto has been converted by all countries, is the bank no longer needed?
I don't think as far as you can predict if an entire country can convert cryptocurrency then fiat money will be destroyed, it's not easy because not everyone can use cryptocurrency and can access a good internet network especially in remote areas and besides there will be pros and cons in government to replace the fiat currency completely with cryptocurrency and even though it will happen in 10-20 years from now but it is not easy.
cryptocurrency only applies as a world currency and paper money as the country's currency and both will continue to be used as any transactions.