Most of the country isn't Seattle or Hong Kong or SF, hemmed in on all sides by mountains and bays. In most of the country, the city could just choose to build more city. All it needs is infrastructure, rezoning, planning permission. And the ability to forgo treating single-family-home subdivisions as immortal, inviolate monuments to the American Way.
The megacity of >10M people is the basic functional economic unit in 2025, the minimum healthy employment market, the level at which we can provide a reasonable opportunity for productive jobs in a specialized role and a reasonable opportunity to hire someone in a specialized role*; Their largesse is taxed or remitted to cover the cities of ~1M, the cities of ~100k, and especially the towns of ~1k-10k.
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*The example a number of economists like to bring up is: If I'm a skilled sushi chef, how long would it take to replace this employer with a better one? How long would it take them to replace me? A thriving economy is an economy that ensures lots of mutually beneficial employment arrangements, in which no one feels trapped, and in which bad management or bad work is punished with replacement, but also which has the slack to absorb random things like interpersonal conflicts or an employee that needs to move for family reasons.
If there are a hundred sushi places within commuting distance, probably at least one of them is hiring. If one chef gets hit by a bus, the business can be back in operation the next day by poaching an apprentice a few blocks away for higher pay. There is always reserve capacity waiting in the wings, as a megacity encourages economic resiliency.
If there are only two sushi places within commuting distance, and I sever my relationship with this one, the other one is probably not hiring, so I am a slave to their bad management and conversely they are a slave to my bad work because it would be so difficult to find another person like me. The quality of goods and services provided to the general consumer suffers significantly, the material precarity of my life suffers significantly. Things become brittle - if the business goes under for random reasons, odds are pretty good that my town becomes a town without sushi. Even if everything is working perfectly... what's my leverage as far as pay raises? What's their leverage as far as work output/quality? We're stuck with each other.
Outside of a city the sushi chef could own the restaurant themselves, including the building itself free and clear. There's less resilience in the labor market, but more resilience from things like recessions cutting the restaurant's revenue (it's much easier to come up with property taxes than rent). There is a virtue in the more responsive economy where a downturn in the demand for sushi results in a portion of the restaurants closing, but there is also a virtue in the less responsive economy where the sushi chef stays being a sushi chef.
> The megacity of >10M people is the basic functional economic unit in 2025, the minimum healthy employment market, the level at which we can provide a reasonable opportunity for productive jobs in a specialized role and a reasonable opportunity to hire someone in a specialized role
Sorry, that's nonsense. Smaller cities like Nuremberg, Ingolstadt, Augsburg etc. are perfectly viable on their own. And frankly, I can't imagine that the numbers are that much different in America.
The key thing is, the rents and housing costs in general in hyper-urban areas need to be paid for by the inhabitants, which means that their labor costs have a certain floor (ignoring assistance programs). That in turn makes hyper-urban areas less competitive in a global market. You can't really compete with Romania for developers when German developers cost twice as much or more than Romanian developers (which are equally capable), and a lot of that price difference goes to the greedy German landlord caste.