Bitcoin is unlikely to ever be the only global currency that will be used worldwide as a means of payment. First, with such volatility, no state wants to completely switch to the use of bitcoins. Secondly, in order to have a common currency, even a digital one, states must conclude a certain agreement, and for this, there should not be political, economic or other conflicts between them. As we can see throughout the history of our entire civilization, this has never happened
I agree that with this level of volatility no one is going to use Bitcoin in the real world. But I can't agree that there must be an agreement or covenant between states. If Bitcoin or some other crypto gets widely accepted in a lot of countries, it will become a worldwide means of payment on its own. You don't need to sign an international treaty with a foreign party to use a cryptocurrency simply because it knows no borders.