The Bitcoin is never going to replace the Dollar. The USA will never let a other currency take over there currency. It can happen if Amerika wants to take over the Bitcoin but it is not going to happen if the Bitcoin wants to take over the Dollar.
Bitcoin can never over take dollar, not even in future as dollar is considered to be the king of currency and its not easy for bitcoin to replace strong currency like dollar.
I think bitcoin and the dollar are two different things, possible future uses bitcoin only limited use of credit cards at the moment, but might overpower popularity dollar