Investors in second (and third, fourth, fifth...) lien mortgages knew that they were subordinate to first liens. Such investors bet that the higher rates paid to such mortgages more than compensated for taking a first loss position. They bet wrong.
People in a position to know such things tell me that one of the impediments to private renegotiation of first lien mortgages is second lien mortgage investors. If there is a place we could use a reckoning, it would be a recognition that such liens have been wiped out.