If borrowers can't control themselves, then the lenders should. The crash happened because of untested CDO's (broadly) and at the heart of that was people borrowing vast sums against worthless properties. These people very promptly blamed the bankers - when really it was their fault for borrowing more than their properties were worth.
The properties weren't worthless when the borrowing was taken out. Affordability was the issue, not equity ratios.
