Paying one cent less than is due constitutes a default. This is what people don't seem to understand.
The US defaulting doesn't mean all the bonds disappear. It doesn't mean nobody gets any of their money. It means the US doesn't fully meet its contractual obligations.
Scale this back: You only pay half of your mortgage payment one month. That means you're in default. Does your obligation disappear? No.
Were the US to default, the important questions are "By how much?" and "How long will it take to fix this?". Only once those questions are answered can you begin to answer the question of what other entities' debt ratings should be.
EDIT: Not all bonds would go into default at once, either. The US could pay some in full and not others, and could pick and choose which got paid how much. Country X that only has a limited amount of US debt might get shafted, while country Y that depends heavily on US bonds gets paid in full. What's the actual consequence there? Not much. Country X is mildly inconvenienced, Country Y has no problems.
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.
The US defaulting doesn't mean all the bonds disappear. It doesn't mean nobody gets any of their money. It means the US doesn't fully meet its contractual obligations.
Scale this back: You only pay half of your mortgage payment one month. That means you're in default. Does your obligation disappear? No.
Were the US to default, the important questions are "By how much?" and "How long will it take to fix this?". Only once those questions are answered can you begin to answer the question of what other entities' debt ratings should be.
EDIT: Not all bonds would go into default at once, either. The US could pay some in full and not others, and could pick and choose which got paid how much. Country X that only has a limited amount of US debt might get shafted, while country Y that depends heavily on US bonds gets paid in full. What's the actual consequence there? Not much. Country X is mildly inconvenienced, Country Y has no problems.
This is way more complicated than 2+2=4.