It's probably something that would not happen in a few years, but I believe there will come a point where the number of users, who will realize the benefits of using Bitcoin, will increase at a rate never seen before.
It is very hard for bitcoin to replace fiat for a very short time and even a decade or two is not even enough for replacing it. We may not when that time will happen that bitcoin will replace fiat or else it will be loved by the people all around the world and it will be possible if many users will have a petition to the world government to consider bitcoin.
Absolutely its really hard for bitcoin to replace fiat because its been used for several years and whole world uses that for currency exchange using fiat always. Fiat is very important in order for you to convert your bitcoin into cash form of money out of digital currency but its good to know that if in the future technology will adopt this bitcoin to be used for transactions particularly on buying and selling not just online but on the stores out there on your place.