It's quite funny how some people like to ask unreasonable questions. Trust me, talking about one world currency is a stupid thing and I don't even know why anyone would be coming up with such an idea. Just look at your question, a currency that doesn't need to be converted but has a crypto and fiat fusion? That's not going to happen. If you feel like you need to be making use of cryptocurrency alone then you should consider making use of stable coins and no one is accepting those now, people just make use of them for storing their their coins and nothing else. So if you make use of stable coins you're not going to find places where they are being accepted, like for e.g in an online shopping store.