Governments are not going to give up the critical aspects of economic control and self-determination that come from national currencies, generally. They could already do that with gold, and they overwhelmingly have had absolutely no desire to do it (for obvious reasons). If Bitcoin gets a lot larger and a lot more stable you may see an occasional small nation, in the midst of economic crisis, peg itself to Bitcoin; whenever the confidence crisis wanes, they'll promptly unpeg to regain flexibility and control.
The extreme majority of nations will strongly prefer to control their own destiny in terms of currency and retain the vast flexibility and power that comes with that control.
One might as well expect nations to give up militaries in the name of peace. That's a similar fantasy outcome that no doubt many millions of people believe could possibly happen (it can't).