I honestly don't think it'll happen, certainly not in 15 years... In 100 years maybe?
Hahahaha 
But honestly, I do believed bitcoin can replace dollar but in this generation. Just like what you said 100 yearsthat could be possible to be happen but still no one knows what will happen in the near future.
I don't think that it will ever happen in future as dollar is the most strongest currency we have and it is more popular then bitcoin and the fact is dollar is widely used and accepted currency and even if bitcoin becomes more popular in future it will never have that power to take over dollar in terms of currency.