You make good points, but the comparison to a mortgage payment is not one of them. My missed mortgage payment affects my credit rating, but not the credit rating of the bank who holds my mortgage because they hold thousands of other mortgages. Despite the bank's leverage, my mortgage is still a tiny portion of their balance sheet. I could stop paying altogether and no big deal. Not so with US Treasuries. They are the foundation for basically every conservatively managed balance sheet. It's the old saying: if you owe the bank a million dollars you have a problem. If you owe the bank a billion dollars the bank has a problem. I agree this is way more complicated than 2+2=4, but Standard & Poor's inability to do even basic math (http://uk.reuters.com/article/2011/08/07/uk-usa-rating-sp-wh...) leads me to assume they are incapable of figuring out the network effects of a US default of any kind or degree.