of course bitcoin can't replace fiat. because there will be people who don't know use internet, bingo, they're not able to use bitcoin.
Yeah, majority of them are not aware about the bitcoins, so it is difficult to expect that they would start using bitcoins, as they find fiat very easy to use, bitcoin can be bit complicated for them.
It will just take some time for them to realise bitcoin. In a few years, the bitcoin user will increase to 10 million people.