Bitcoin can never substitute for traditional fiat currency.
It was designed for different purpose and can hardly fit requirements put forward to fiat currency. If anything, there should be some new Cryptocurrency that will fit society needs and it will not be Bitcoin.
No, it cannot replace fiat, but I can admit that in 5-7 years, the cryptocurrencies will be used ion most places, including the land stores (not online shops only). For me, it will be very convenient.
Without Fiat we can not know the true value of bitcoin. Fiat make us know what bitcoin price is. And the best bitcoin can get to is to be a competitor to fiat and not replace it.