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 do not agree that bitcoin will replace the fiat. I'd rather see bitcoin as an option for the fiat itself. Although the potential for bitcoin exists to become a global currency. But I prefer bitcoin to be a currency option. Probably for the stability of the world