We don't speak the same language, we don't eat the same things, we don't even drive on the same side of the road... How on earth would the whole agree on 1 common currency?
Not to mention that there are powers that thrive on conflicts and actively antagonize various groups within communities, nations and worldwide.
The problem is that people here think too far ahead. Real scenario is that Bitcoin can be used as a global currency at any time if people so wish already, so there is no need to wait for authorities and whatnot to stand up.
We just have to accept that Bitcoin will probably remain just an alternative, which I am perfectly happy with. While people are discussing unnecessary matters, I focus on what I can do with Bitcoin right now.
Fiat or global currency doesn't play a role of importance in that regard. Action is what makes people move forward, not endless nonsense discussions about what happens in this or that scenario. Bitcoin is here and ready to be used.