World governments are cyclic events. Every few hundred years, a new world empire rises, and it eventually has a revolutionary downfall. The Egyptian Empire, the empire of Alexander the Great, the Roman empire, the British Imperialism, etc... It's happened before, and it will happen again.
There hasn't been a world empire since the Westernization of North America. (The final death throes of the British Empire occurred only shortly after the formation of the U.S.) But, I think we'll be due for another one here in the next 2 or 3 hundred years...
Of course, I don't think many of us will live to see it...