It all sounds wonderful, but here's why a one-world system would never work (leaving aside differences in religion, culture, language, and so forth). What happens if you end up with a dictator in charge? Where do you flee, if there are no other nations to take refuge in? The moon?

With human nature being what it is, we need boundaries and different nations to keep the bad ones in check. I know this isn't always done effectively, and that there have been (and are) horrific wars fought between nations, not to mention racism, xenophobia, and so forth, but the alternative - a potential global tyranny - is unthinkable.