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.
Bitcoin is not able to replace dollar because of its natüre. You can add new features to bitcoin. That time, maybe it can replace dollar but America does not let this happen.