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.
The actual fact is that every nation has its own currency and they love their currency and work for its establishment, so they will stay will their currency and will use bitcoin as a side currency.
If anything that side currency, is just something they can trade back to other foreign currency.
Which acts more of a debt prevention, since its being universally shown value.