I honestly don't think it'll happen, certainly not in 15 years... In 100 years maybe?
You got a point there, dollar has been around and ruling the world for how many years now, I think its not gonna be replaced by bitcoin or any other cryptocurrencies that easily, 15 years is quite fast, bitcoin is being known world wide in a fast rate, but the legal matters of it is gonna drag cryptocurrencys popularity, so I think its not yet gonna happen anytime soon.