Yes l agree with you mate we can not totally eliminate Fiat currency from financial transactions despite the world is turning to a global village where digital currency will serve as means of transactions the usefulness of Fiat currency  can not be under rate. There are still people who don't speak or write or even know the internet of things, these set of people are the once we some time buy raw materials from in the manufacturing industries and other sectors as well ,because of their inability to computer or phones they will prepare transaction such as the physical Fiat currency they can see  and this might slow than the rate of production in businesses.
My suggestion:
these people should be more educated and teach on how to use the crypto currency as a fastest means of transactions in their  dealings. If this is address and achieve the crypto currency will result heavy mass adoption.