I honestly don't think it'll happen, certainly not in 15 years... In 100 years maybe?
I don't think that bitcoin can replace dollar after 15 years but who knows it might happen if not in 15 years by now,maybe after 20 years or more..It's just only my opinion.
Don't think it would be possible to happen because dollar is a different currency compared to crypto currency
and dollar is well regulated and has been created and supported by the government, I think this is impossible to happen so let's stay realistic all the time.