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.
I think even 100 years even though it will not make the fiat replaced by bitcoin. I think during the reign is still a role model. Then bitcoin will not be able to replace bitcoin. Because the government will definitely continue to use their own local currency