Agree. Not all nations have the technology to implement the cryptocurrency system in their economy. So I think that the currency of any nation will remain in its finances. If there will be the handling of cryptocurrencies in cities that have better technology infrastructure.
I have seen it in my country. When the banking system implemented mobile payment and not all citizens have access to a smartphone.
This problem may also be encountered by those who have a Bank card at a time when there is no electricity and there is no possibility to use the terminal. Therefore, in addition to a credit card, everyone should have a Fiat. And you correctly said that in order to make a payment in cryptocurrency, it is necessary that everyone has a smartphone to make the payment.
Not only about smartphones because even for the use of transactions in digital form requires internet access and in fact, there are still many constraints about these things. At least there are still advantages and disadvantages of digital and cash payment methods and as such, at least fiat is indeed not irreplaceable but on the one hand, cryptocurrency and or other digital currencies also have their own options for those who prefer digital or virtual methods in an era that modern.