> I’m concerned, of course, that Americans have built a society with so few buses that everyone has to buy a car on credit. But being “underwater” is not the main problem here.

More specifically, we've built cities that are pretty awful for bus (or rail) transportation, if you thrown any sane amount of money at the public transit system. Most of our cities have been relying on inefficient (i.e. very spread-out, low-density) growth patterns to cover costs, while the long-term expenses demanded by that growth (maintenance, for example) outpace the revenue it brings in. It's musical chairs, and when the music (growth) stops most of the cities in the US are going to be Detroit, with way too much city for their tax base to support.

Huge, low-density cities lead to shitty, expensive services (bus service being but one example) and crippling, indefinitely-ongoing maintenance expenses.