Bitcoin cannot replace dollar. Why? Because dollar is a powerful currency. It is hard to be changed by any currency. Besides if bitcoin replaces dollar, there will be a country that owns bitcoin as it's currency.
In fact anything is possible in this word. Even bitcoin can replace bitcoin, but it is not such an easy job it will require that all the people of the world start using bitcoin, and i think still less than 2% people know about bitcoin.So it will take a lot time when all the people of the world will start learning about bitcoin.