Yes bitcoins can never replace fiat as the user of fiat are much higher in compare to bitcoins and even in future majority of people will prefer to use fiat money as they are very much used to it and find comfortable in using it.
bitcoin is much comfortable than fiat, people only need to understand and know about bitcoin. than they'll see how easy and smooth using bitcoin is.
right now higher amount of population using fiat, but one day this scenario can change,