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.
In whatever way bitcoin is spread, it wont completely reach the entire population. So some don't care of bitcoin, they'll just use the fiat.
Yes people who find it easy to use fiat will only use fiat and people who loves bitcoin will use bitcoin but that doesn't mean people will stop using fiat.