Fiat will always be in demand and it won't care where bitcoin reaches in the future, people who are comfortable using fiat will use fiat only and not bitcoin.
but if bitcoin prices become very high. bitcoin will certainly become popular then many people would be interested in trying bitcoin I'm sure of it.
but I think as gold. bitcoin will still be commodities or payment option.
Fiat will remain worthwhile
If bitcoin prices will go high then also people will convert it to fiat to hold more amount of fiat which will take their buying power to a higher level.