in my opinion , that will never happen . see, if they actually did that , it will have serious implications on major trade currencies .For instance , US dollars and american economy in general gets its strength from their currency usage in global trades . once that replaced by bitcoin or any other decentralize crypto currency , governments will lose any privilege in their economy and they won't let that happen.
Well, you have a point but yet, this will still happen, bitcoin will soon affect the World trade and it has started doing so too. Bitcoin is a form of money, a digital currency which are been used by two parties on agreement to pay for goods and services and any currency that are accepted by people as a payment method has the power to affect the World trade. Even though the government may not like this, they can't control it to some extent.