Bitcoin will not really replace dollars or other fiats that easily, not just in 15 years but in 100 years. Bitcoin is a good currency for the people, but with it's current state now, I don't think it will be a primary currency of one country. And if bitcoin will be a primary currency, replacing fiats, do you think all the people can keep up with it? Not all people are that updated especially about technologies.
Absolutely it won't replace dollar in our system of currency because fiat currency is so stable and at this time most of the people and businesses relied with dollar. It's so hard to let digital currency be at the system that exists from many centuries, specially that technologies of bitcoin can be so hard to adopt to people who are not yet open to it, and they are mostly hesitant to know. So if there's a plan of the Government to implement dollar fiat currency be replaced, well they will finance most of the lower class people; they will be first to be affected with.