That will not happen in anyone's lifetime, trust me, no one will want a single currency to be used all over the world let alone a digital one. The reason why is because some countries will insist on the superiority of their currency over the other one, I mean look at US dollars, they're still contested to this day by China as a ruling currency for international context.
Geopolitical power is definitely a big factor here, but the biggest one still has to be about financial control -- big economies are able to freely manipulate their currency to fit their needs, and Bitcoin will not let them do that. Even just adopting it as an alternative legal tender would undermine their control because their fiat would be seen as less necessary.
Bitcoin being a global reserve currency just seems inevitable at this point though.