I think that's a failure of imagination, and that vision isn't one that history necessarily supports. Humans used to exist in small roving bands. Now we are organized into nation-states. I don't see a truly compelling reason why that trend towards greater organization can't continue. I am also certainly capable of imagining a world government that is responsive to the needs of people, maintains global peace and security, and focuses the vast majority of its budget towards constructive, not destructive, ends.
I recognize that this doesn't look likely in the immediate future, but:
- An alternative consisting of warring nations equipped with civilization-ending weapons is surely not the only path forward (it better not be, if we hope to stick around).
- We are increasingly faced with existential problems that require a globally coordinated response.
- Change often happens a lot faster than we think possible. The order of things is set in stone until suddenly it's washed away and everything is different.