is bitcoin going to replace the us dollar?

That's highly improbable. According to the IMF 63% of all reserve currency is comprised of the US dollar (Euro coming in second at a quarter of global reserve currency). Bitcoin has a LONG way to go before it can even be considered meaningful competition.

Another reason why Bitcoin won't replace the dollar is because moderate and conservative investors are hesitant to jump into this new fad. If you manage a multi-million dollar portfolio you're going to want some proof that Bitcoin isn't going to be a passing fad. The dollar has been a stable currency for decades, whereas Bitcoin just came into existence. There will be risk takers who will jump right into Bitcoins, but overall the wealthy investors will probably remain skeptical.

Bitcoin also isn't backed by a government or central bank, which presents a series of other problems.