Bitcoin will never replace fiat. Bitcoin have their own target and it's digital currencies. If bitcoin have physical unit like fiat, maybe bitcoin can replace it.
Bitcoin never replace fiat money even bitcoin have physical unit like fiat, the problem is which countries would adopt bitcoin to replace their own currencies.
i doubt that this opinion is correct to be honest, in my opinion the bitcoin price will grow really much in the near future thus allowing people to make money and it will replace the fiat
i think that its the only question in time when it will happen and we will surely see some good changes in my opinion and after a few decades we will use bitcoins instead of fiat