There are systemic shocks that you're sweeping under the rug.
If most people in a state is underwater on their house and they declare bankruptcy there is a real economic erosion that occurs. Foreclosed houses get looted, depressed people stop working, already stressed state finances break under the burden of further aid applicants.
By saying they'll just "modify their behaviours" you're essentially saying they'll just "go on welfare or become homeless on the street".
Most of America is not ready for the next recession, and it is coming at the worst time, when Baby Boomers are starting to strain the retirement and healthcare systems.