We made bad assumptions regarding default probability. Analysis of internal documents has been done - even the "pessimistic" estimates were far less bad than what actually happened. That would have been a problem regardless of securitization.

Securitization didn't fix this risk, nor was it meant to. Regulators allowed banks to satisfy capitalization requirements with senior tranches of other banks, but not with the senior tranches of their own portfolios. This isn't necessarily a terrible idea - it reduces local mispricing risk (i.e., one bank's underwriters suck), just not global mispricing risk (everyone sucks in the same way).