It won't reduce all consumer debt.  Credit cards will just boost the interest rate.  The only way you come out ahead is with fixed-rate debt.
Yes - one of the questions is just how far over-extended are most of the people who live in the exurbs?  I often have the impression of people working 2 jobs long distances away, and leveraged up the wazoo with variable rate debit.  But I don't know how much of this is really true.

Also, if we run electric light rail lines to these communities, they will remain viable to some extent.  I would expect them to eveolve into actual towns if they are to survive.

If the dollar collapses overnight you don't need to worry about credit card debt. We are in completely uncharted territory.
The most dangerous words in financial booms are "This time it's different!". But the bust after the boom is always different. Compare 1900+, 1929+, 1966+, and 2001+. Each was different.