Bitcoin can no way replace the fiat currency. Fiat currency has got its own advantages. Just imagine if we want to purchase a 2 dollar object from a shop, we always try to pay the shopkeeper with fiat currency. Suppose if we pay in bitcoin, we need to spend totally 5 dollars for purchasing it including its transaction fees. Bitcoin is useful for bigger transactions and for smaller transactions still fiat is the best method.
if transaction price will go down, crypto currencies can replace credit and debit cards. So yeah, there is the way Bitcoin can replace fiat - become more convenient for small payments. That's it.
Some companies are ready to adopt cryto currency, since it's fast and more cheaper then for sure more people will adopt and their market will increase.
I don't think even bitcoin will be popular it will replace fiat, think of the present situation, people are still not willing to take risk without government regulation.
Bitcoin is still young and there will be a lot of battle or trial that it needs to surpass.