In my own point of view it will not be. Why? because there is Federal Reserve, why I say so? because think of it if bitcoin will be the source of exchange worldwide then where do the biggest financial community in the world reside? I think it take a lot of time before money will be lost in and bitcoin will be prominent in exchanging some goods and services. I think money and bitcoin will be hand to hand in handling the most used currency in the world in the future.
Fiat will not be easily disable or of no use instantly. It will take time, so much time in my opnion. Bitcoin and fiat can exist together and bitcoin is a good alternative and poised to become mainstream in the future.