The currency thing is much of a muchness in the grand scheme of things. The main thing is that you have good trade in the first place
The kind of wealth and prosperity we have right now is incredibly fragile. Right now you have things like the Ukraine war and COVID shenanigans in China threatening the globalisation that allows the supply chains that allow the kind of trade that allows the wealth and prosperity we have
As you start to see food, energy, supply chain and ultimately financial crisis, it doesn't matter what currency you're using - you're far worse off than you were before
And currencies need to have backing. Even decentralised currencies need enough backing from enough people to be considered valuable enough to remain a currency. How do you enforce one world currency when you have so many competing nations and groups?
Say some country made a crypto that was just the perfect currency and it started seeing worldwide use over the US dollar. What if that country got into some war where some other countries then laid siege to their shipping lanes and that country wasn't able to import the food and energy needed, and the nation basically failed and stopped being an industrialised nation? What happens to that one world currency then?