The suburbs will maintain their own roads before the bottom falls out. low income suburbs probably will not survive, but there's no law that says they cant have shitty roads that require a jeep to drive on
when the roads require a jeep to drive on, it's no longer a suburb - it's the country. the promise of the suburbs is that it's all the luxuries of city living, with all the space of rural living. Losing the amenities of the city is exactly how i would interpret "the bottom falling out"
Middle- to upper-middle-class suburbs will still exist, they'll just be a municipality a few miles over, where all the houses and roads are 25 years newer, and the long tail of opex costs hasn't quite hit as bad yet. It will, eventually.
If a suburb can attract some retail or office space or manufacturing, they can be like Irvine or Tysons or Bloomington -- the suburbs that have made it. Those that cannot will accumulate the same problems with fiscal sustainability that inner cities once did.
High income suburbs will struggle, too. Gotta pay for the roads somehow if federal subsidies stop trickling in. People on HN talk about moving to texas on taxes alone all the time.