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.
Exactly, fiat cannot replace bitcoin int the future, although I can say that in the future their will be a massive adoption of bitcoins as long as they will be educated and will understand the benefits in using bitcoins transacting online.