The replacement of fiat with bitcoin wont happen, just compare the volume of dollars at forex or any other currencie and will see that bitcoin its just a portion from the total, very tiny. The future should be coming over the next 50 years printed money will became digital as bitcoin nowadays soo its a great combination have both.
Both are different currencies and cannot replace each other, and if have opportunity to use both them then why to choose single one, use it according to your convenience.
Yes its not possible for bitcoin to replace any paper currency, as the market cap of fiat is too huge and comparatively bitcoin is too small, and Government will never permit bitcoin to get on the mainstream.