Bitcoin is not here to replace fiat, bitcoin is here to help our in our lives.
My hope is that bitcoin will someday be the main currency for transactions.
That would be the best.
Bitcoin will be used as a payment method for sure but I don't think that bitcoin can replace fiat, as the users of fiat are more as compared to bitcoins, and people don't have faith in digital currency, as they are not familiar to it.
I think it will just take times. It might take centuries to replace the fiat. The society will be cashless eventually.