You clearly do not understand securitization structures.

You need to look up the concepts of tranching, equity piece/residual valuation, all in spread, overcollateralization, IO vs PO, yield expectation, prepayment pricing, to name a few. To think that a default in an underlying loan (or 15% defaul of underlying loans for that matter) would necessarily lead to a loss to structured debt investors is a clear sign that you do not understand the structure, pricing or market movement of these instruments. Yes, losses do happen, and it could get very bad very quickly. But not in the way you describe, and not to the party you described.