chances are very small. fiat has advantages in terms of direct payments and real value. digital currency will be useless when there is no internet reach. it is certainly very detrimental. so, the existence of fiat is still very bleached.
Although the chances are very rare that bitcoin will replace fiat, but i think that with increasing demand of bitcoin is in fact making bitcoin more stronger. i think that in such modern time it is very difficult to think about the world without internet, because internet is not only use for bitcoin for a number of other purposes. so we cannot spend a single day without internet. I think that its difficult that bitcoin will replace fiat but will surely reduce the use of fiat currency.
No doubt that bitocin is so much stronger currently but it cannot disappear fiat completely across the world because there are so many countries in the world who dont know even electricity and unaware of computer technology but a time will come when mostly people will be dependent bitcoin and generally we will be able that time fiat is going to end.