Fannie and Freddie will not fail. They are too big to fail, and everyone knows it.
Their loans may turn out to to be worth hundreds of billions less than what was loaned, but then the govt will step in to save them. Over time, as these two GSEs loaned more and more in overpriced markets, backed by private capital that is a minuscule fraction of the loans, they went from being arguably partly private organizations to almost totally government organizations. Now the private capital they have can absorb only a tiny hiccup in the hyper-inflated bubble market, and the private label is a charade.