I think that this will never happen. The dollar is the currency that the government supports. That's why the dollar will always exist. The government can not trust Bitcoin.
Most definitely not, it may seem that bitcoin is a highly popular currency and we are comfortable to use it but on a large scale it cannot be used by all the people in the world because it cannot cover all the needs of the whole world, I mean right now probably not even 1% of the world is using it and we are struggling to keep the bitcoin network stable.