i do agree that bitcoin cannot replace fiat. and that fiat cannot be completely eradicated and i don't see it happening anytime soon especially now when bitcoin is still under development.
but i do have hopes in the future that it can be used in daily life as easy as paper money.
I also believe that fiat will still be used. Many still do not know about Bitcoin. It will take much time to until Bitcoin becomes equal to a centralized exchange
Some might know bitcoin but we cannot expect that they will be interested with bitcoin, the fact that bitcoin is only good for online transaction it has only limited features, fiat are both for offline and online transaction so fiat is more dominant than bitcoin and it cannot be replace by bitcoin.
How about if in future we will do anything online that would use fiat reduced not later bitcoin could be one of them because with bitcoin transactions can be confirmed fast.