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.
And many third world countries will have problems accepting something that requires WIFI and internet connection..We must fight for better human life conditions all over the world before we place such high demand. But bitcoin is just money don't care for people.
Many places here in my country has no wifi or internet so its not recommended to replace it.. also some ignorance like igorot they didnt know it so its impossible that can replace fiat..