previously, fiat currency is popular when Bitcoin wasn't familiarized with people, Right now it is equally useful to convert to any transaction anytime. I think almost all countries using this except in areas where gold is more priority, although fiat is very risky from where it is used for the short term. as long as you have more, instantly, through these sources, you can buy Bitcoin for trading, and it is valuable towards the good profit within this year.
But the fact that Bitcoin is not ready to be used by the government will make it impossible for a world without a fiat currency. We all know that most of the infrastructures and projects are made because of the taxes from the people, and without fiat, it is really hard to regulate the economy of the country, right now, a lot of government are making own way to still adopt cryptocurrency while their fiat is still strong and worthy. Anytime soon, countries might transition to cryptocurrency but still, it will be classified as fiat as it will be regulated. I think that is much better since it will provide transparency for every transaction as well as improve the speed and efficiency it.