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.