Have always wonder which currency would the world accept if only one currency was to be used. Maybe the dollar?, don't think so, The British won't tolerate that.
Cryptocurrency?. Maybe. Until the advent of cryptocurrency, I never thought that would be possible but it seems possible.
The world is not too far from a unified currency.
No, in order for a single currency to exist in the world, there should not be many states. all the more so with different political systems and other factors that separate them. A single currency is possible when all states are united into a single superstate, or when they are closely united with a generally recognized single government.
However, decentralized cryptocurrency is not suitable for this role for many reasons. It could be one of the future regional stablecoins of states or the stablecoins of a major state.