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.
I don't think it will even happen in the future, dollar is a big currency and we cannot beat it.
The approval of the government is already a big news for bitcoin and makes the price rise, so what will happen if there is a bad
news regarding the actions of the government, it might become worthless.
What i believe is, dollar or any fiat currency will stay as well as bitcoin because they can't eliminate someone because they have their own purpose and we should respect that and as what i said, bitcoin has its own purpose and dollar can't do much about it so they will either work separated or they will work together to improve their selves at the same time.