There will still be some people who use fiat currencies.
Would you like me to mail you a letter with more thoughts on this?
Or you could read my editorial in the newspaper the next time you sit down and flip through the pages.
If Bitcoin ever did become mainstream do you really think fiat would still be used?
It almost certainly will. Governments will always force it's citizens to pay taxes to help pay for things like roads, national infrastructure and national security. The government will be able to exert it's influence to both pay for these things in fiat and to collect tax revenues in fiat.
It would be possible for bitcoin to replace fiat in very small countries when the government does not have a lot of power (due to the economy is very small), but bitcoin will not be able to replace any major fiat currency.