To replace the dollar with bitcoin in the near future seems impossible, we all know that if the USA government is angry it will do everything possible to destroy bitcoin. I think it would be difficult to replace the dollar as long as there is no strong country that legalize bitcoin.
Although I also believe in bit coin and love it, I don't think it can replace dollars not only in 15 years even in decades. Gold can be a good example as it is valuable against dollar but they are not being used to buy oil till now. Its how us dollar is dominating world market and their is a huge role of government for it. So I think same thing will happen to bit coin like gold.