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.
They will reach all population of the world. Try to imagine bitcoin as a fiat. Fiat is well known money. Bitcoin might go there too if our technology will concquer all places of the world