That day will never come, its impossible for bitcoin to replace dollar ever in future, dollar is the most popular currency worldwide used and accepted everywhere. It will hold its power in future too.
Slowly and gradually the day is coming nearer. Although its look impossible but anything can be possible and very soon that bitcoin will replace Dollar, at least it will reduce the use of Dollar. Because in modern time i think that all the people of the world will give important to online currency as compare to fiat currency, which will then become an old idea.