It would be very impossible since I don't think the government would want to lose control on their citizens. Bitcoin can be easily hacked. The government could possibly lose alot of money if they get hacked. But it could happen if the current government of every country is squashed.
The advanced technology is very successful. Anything can happen in the future. So it may be possible that governments make some regulations through which they will be able to handle bitcoin and will be able to maintain the affairs of the state with the bitcoin currency so it is possible that bitcoin will replace dollar soon.
The government can regulate the usage of bitcoin but not as a currency that will replace their own.
USD is their representation and they can control it, imagine you replace it with something you are not in control, it does not make sense.
Yes that is truth. American dollar is something like part of their personality, it is hard to change that. And even if they wanted it would be hard because there are so many things that needed to be done before bitcoin could be used in day to day business. Maybe it would be possible for bitcoin to replace dollar, but it will not happen any time soon.