Freddie and Fannie are the nations biggest money laundering operations. If they failed, all kinds of garbage would come spilling out, and that just can't happen.
Our national debt is not money that our country owes to itself or a meaningless number like a lot of people think, it's actually the amount of money that the U.S. owes overall to other countries in the world. Each year the U.S. government borrows billions from other countries like china, Russia and many from the E.U. and pretty soon the interest payments on our loans are going to exceed the GDP. The question is not whether our economy will fail, but rather how the rest of the world will react when we start defaulting on our debts to them