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.
No doubt about that.. those old people who are using fiat and don't want to open up to new technology would always use fiat, even us, who are collecting bitcoin, we still have that instinct to convert our coins into fiat when the price is good..so never that fiat will be worthless even when the price of bitcoin worth thousands..