What I believe is that fiat will always be there, one way or another.
In my understanding, cryptocurrencies were never meant to completely replace fiat money. It always be just an alternative.
Exactly, we have to accept the reality that bitcoin is just an alternate currency even in the future. Fiat is a government regulated money that is acceptable all over the world while bitcoins is not that popular and still has a lot of government issues that needs to sort out.