USD etc has such a big foundation that it's near impossible to knock it off it's throne if you ask me.
I do believe also that fiat money will be replaced by a digital money that is just like going to be implemented first by US government but I don't think if they are going to rely with bitcoin. It is going to be cashless society and that is going to be implanted with the use of a microchip and that is going to be your wallet and by just scanning it to supermarkets you are going to pay your groceries conveniently and fast.
Now imagine how the limited supply of coins can support all people in the world. Since bitcoin will replace it's money it must surpass the total numbers of printed money by every country. And take note, some will hoard bitcoin so it's not making sense to believe that bitcoin can replace money.
The good thing here is, we have many options to choose. Why bother replacement if we can get benefits from both of the party.