Unlike the hunter-gatherer concept, medium sized cities can work. But they don't.

Cities in the 50,000 to 250,000 population range are not doing that well. That's what drives population to mega-cities. This, when you think about it, is s strange. A city in that size is big enough to support multiple factories, large retail establishments, fully equipped hospitals, entertainment businesses, repair services for all key technologies, and an airport. So why don't such cities outperform mega-cities, where the friction costs of scale start to get in the way?

This has been studied. Search for "medium sized cities" "problems". The phrase "urban shrinkage" is used in Europe. In most of the developed world, medium cities are not doing well. What seems to happen is that young people leave, the average age becomes older, a smaller fraction of people are in the workforce, factories close, and a slow death begins. Those forms of decline feed each other.

Japan is furthest along this path. Only Tokyo and Osaka grow; the rest of the country has hollowed out. It's a big issue in the US and in Germany. Don't know about other countries.

Without growth, medium sized cities age and die. Yet population is leveling off. Now what?

With the difference that the larger the cities, the less we know the people around us and socialize with them. Totally different if you live in a tribe of 200 people or a mega city with 20 million people around you.

Even for the social butterfly, the number of others someone can actually communicate with is capped at a reasonably low number.

Yes, but the size of community matters. In a 100 people tribe everybody knows everyone else. In 1M city most people are strangers, a.k.a. competitors, enemies even.