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.
It is true that Bitcoin will be used world wide in that time but it will not be substituted for thesw fiats. I think this is the reason why a lot of people hates Bitcoin. They think that it will be a substitute but it will never be. For us, Bitcoin is mostly an investment and if you invest, you save them not use them to pay your bills. Being used universaly, that will be great since the demand will be increasing and obvioisly, the price will increase too.