You actually did not give strong reasons why cryptocurrency can't replace fiat currency. Cryptocurrency can actually/theoretically do most things fiat currencies can do, but I doubt typical fiat currencies can safely do most things cryptocurrency can do with losing their centralization. Besides, government can run their fiats on Blockchain or other real decentralized system.
Crypto could actually work better in rural areas than fiat, even without relying too much on the internet. But I believe safe/crypto-friendly worldwide rural internet coverage is a possibility in the future
You have not given convincing reasons why crypto is better than fiat. I suppose you know billions of people are still unbanked not to talk about thousands of billions of people that do not even know what crypto is not to talk of using it for making the purchase. Even a mad knows fiat when he sees it on the road. Rural areas dwellers that do not have access to the internet so how will they make use bitcoin ?. Do you know there are still millions of people in Africa that do not have access to the internet ?