I don't think that it will ever be possible to replace the dollar because the government won't let the dollar be replaced completely. They will find someway against the Bitcoin.
And I am very positive towards the future of the Bitcoin because it keeps growing in worth and the community keeps getting bigger.
Or they will still continue on there Daily operations without even worrying about Bitcoin. When you are talking about Bitcoin it is like you are seeing it as almost a threat to everything, but what you did not know is that Bitcoin can be controlled by the government as much as they want too.
The government can even make a law to make Bitcoin addresses be identified to each users therefore making money laundering hard. A country's currency will not be ever replace by a foreign currency.

They cannot outlaw the law of gravity
In the same manner, they cannot outlaw Bitcoin. Indeed, they can adopt a law that will be set to prohibit Bitcoin possession, but how are they going to enforce such a law? People will continue to use it secretly. Today, when the world has become a tight place, such laws are nor very implementable overall, and that seems to be the main reason why most governments abstain from outright banning Bitcoin. Even communist China is set to drive Bitcoin down by indirect or even underhanded ways and approaches (but it still doesn't work)
The bitcoin can be regulated, but not outlawed.