Never going to happen. Governments are run by controlling things. If you want to replace dollar with bitcoin, you need to hand over the control of BTC to government. How a country's economy can be based be on such volatile thing.
If you see it from other angle, BTC can replace if every online business starts accepting BTC for transactions. They will still need to convert it to dollar.
You have a point. Before thinking on replacing dollar by bitcoin, bitcoin should be legalized first. And yes it means that the government will have the power to manipulate bitcoins transactions and production. And we do not want this to happen because when government have the power to manipulate bitcoin, corruption may exist. All i can see for now is bitcoin will be a choice of people on transaction wise not a total replacement to dollar for instead a way of an easy transaction. But the possibilities does not stop for now because we all know that technology also developing meaning that bitcoin future is more to discover for now.