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.
The thing is that in well developed countries as in Europe have a very advanced payment system where the normal people are very happy with. They can buy stuff instantly with their debit card or via internet banking or simply via NFC. These people don't care about the decentral aspect of Bitcoin. Convincing these people to use Bitcoin will be a real challenge.