> I agree that American society doesn't need any one particular city, but we do need a healthy ecosystem of urban areas.

Do we?

I'm happy to assume the benefit of a couple of finance-urban-centers, but it's unclear that there's a huge benefit from having finance, art, and fashion in NYC. If fashion and art were elsewhere, NYC would be significantly smaller, but would we be worse off?

My point is that the big urban areas in the US contain aggregates that may not be particularly synergistic. If they're not, then its the "lower-tier" cities that are actually important, and that we happen to co-located many of them.

"are the smaller, lower-tier cities necessary in order to have that healthy ecosystem?"

And that question can be rephrased as "Do smaller, lower-tier cities contribute to a socially and economically healthy America?"

Don't get me wrong, I really, really want there to be a reason for cities of 50K-500K to exist. Unfortunately, I think it's too small of a labor market for talented but not self-suffiently entrepreneurial people to bother staying in. The only towns this size that are prospering outside of major metros are state capitals or college towns, which benefit from a large, captive employer.