I think it is impossible for Bitcoin to become the main currency of one country because it oppose the idea of the centralized government. They could use it as an alternative currency though but mainly use it, no it would mess up the current monetary system. Besides, the transaction speed and fees would also matter or could be a problem upon using Bitcoin as a country currency. Bitcoin was decentralized, meaning everyone has no control over it and the government won't allow these things to fully operate without them managing it.