Fiat is the one that has given the best financial service for all our needs. If fiat is not there the system will collapse entirely, so fiat will rule the world even when bitcoin and other currencies increase in acceptance.
you are right that at the moment fiat is the best financial system though in the future all the things might easily change, I think that bitcoin will be a national currency in the future.
Bitcoin is already a global currency, but bitcoin cannot take the place of fiat currency. Fiat currency is ever green currency where we are using it from decades, fiat currency is the majors aspects in human being daily life. Through we have many option where we can use it for all needs we have.
currently it is not taking the place of fiat but i think in future when the people will fully implement bitcoin then it can be possible to replace bitcoin instead of fiat but still it will take a lot of time and it is not so easy.