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 will not substitute national currencies, but I guess that the time, when Bitcoin can be used everywhere will come. There will be BTC ATM machines, and the stores will work with this crypto.
Of course, it will not replace them, but it can be used everywhere - why not, man? People will be free to choose what they want to deal with - cryptocurrencies, fiat or both of them.