There is nothing wrong in using both of them, fiat is much in demand in the interior parts where internet and bitcoin has not reached yet, and it will surely take long time.
This is exactly the point. How come anyone wants bitcoin replace to fiat. There are lots, as in lots of places in the world where internet are not present and never be will present even in future.
Just enjoy whatever benefits that bitcoin is giving us. Don't just think too much unrealistic because you don't understand what will be the output if there's no fiat present in the future.
So what is that "easier thing" you are talking too to support that "unless" statement.
Bitcoin doesn't have an ability to touch the whole world and never it will.
Please remember that bitcoin uses blockchain technology.