The big problem isn’t the underlying mortgage values. The big problem is the Credit Default Swaps that were written on top of it all.
As I’ve posted elsewhere, MBS derivative risk is a convergent series (toward zero) because any deravtive will have LESS risk than the original obligation. And given the premium for off-setting the risk, the derivative risk is generally substantially less.
So first order derivatives are less risky than the original.
Second order derivatives less risky than the first order, etc.