That's a reductio ad absurdum. There are reasonable levels of division well above tribal levels. Singapore is basically a city state, and I don't notice a collapse of civilization there.

Territories with strong social identity would maintain political union, territories with weak social identity and severe political disagreements would split. As much as is possible, no one is ruled by people they hate.

https://en.wikipedia.org/wiki/Subsidiarity

We are taught this is impossible because the ruling class, who favors ever more centralization, wishes that to be so.