Could you see a future wherein the entire world shares one global currency, be it fiat or crypto? If I think it in a scifi kind of way, it would most probably be some digital currency that could all just use. It doesn't even have to be in this century. Maybe the next?
Almost every state has its own national money, which interacts with the economy of this state. Each state seeks to strengthen its currency and with this it strengthens its economy. It seems to me quite normal. The state simply cannot switch to using the currency of another state or cryptocurrency, which will not interact with the economy of this particular state. This will lead to the imbalance of economic indicators of the state. At the international level, states can agree on the use of a single currency, because this single currency will eventually be recalculated as a result of national money. However, the state will not be able to fully switch to the use of other people's money.