Co-existence of fiat currency and bitcoin can have many advantages and many are thinking this is the future for cryptocurrency rather than taking over fiat. Online or digital transactions can be covered by bitcoin and physical exchanges can be done by fiat.
I agree that fiat currency will exist. But I don't think paying for things using something physical is an advantage. Cash is used less day by day.
For me,there's nothing wrong having both of them as legal currencies.People will have more freedom to choose what options they will take,either with fiat or with cryptocurrency bitcoin.I think this will create a more stable economical level as this two are already working hand in hand than competing with each other.