It might not happen soon, or even for current cryptos. However, it's not really a stretch to imagine a unified currency ruling the world in the near future. It will, by necessity, be decentralized since no country will willingly accept another's control over its economy. It has to happen organically, otherwise current rich countries would have no incentives to "allow" competitors on their turf. However, if it penetrates the world sufficiently, the transition would eventually happen because the need exists. It's only blocked by policies.