[timr]

> Correct, but once underwater, an default renders the loan worthless.

Incorrect. A loan in default can usually be pulled out of default, or otherwise re-structured to keep the borrower current. They are not worthless. In fact, there's a whole sub-industry devoted to this called "special servicing". Even for underwater loans, people tend to want to repay their loans.

You are directionally correct that as a loan gets further into default, it loses value, but this is not a step function, and it certainly doesn't happen instantly on default. You're over-indexing on an exceptional outcome from an exceptional time -- even in 2008, the vast majority of distressed borrowers weren't walking away from their loans.