The crazy volatility of cryptocurrencies constitute one primary reason why they can never replace fiat currencies,
Volatility is a short term concern. Most people don't use crypto for payments anyway, which is actually a good thing because it will allow Bitcoin to mature and become stable the bigger and more liquid it becomes.
In Venezuela people were better off buying Bitcoin at $20,000 and ride it down than hold their own fiat currency. Bitcoin despite its 85% correction from top to bottom still did better than their fiat currency.
but there are more pressing factors such as internet usage. Cryptocurrencies existence is mainly made possible by technology and the internet. With a device and access to the internet, almost anyone can get into the cryptocurrency game.
I'm not sure if you're up to date with the age of digital payments, but in most well developed countries people use their smartphones to pay for goods and services. In this case if your internet doesn't work, you can't spend the money you have in your bank account. Not much different from when you don't have internet and try to pay using crypto.