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 internet. But I believe safe/crypto-friendly worldwide rural internet coverage is a possibility in the future
Do you think crypto can work well in rural areas and don't even need the internet? the fact that you convince them to use a new currency and to use technology is something unimaginable. They are people who don't know about technology and you force people to follow?
It will be very time consuming and even impossible to do if the people do not cooperate. I feel that crypto should only work in Defi, it should not be in centralized markets.