I'm surprised that you are an accountant who can make such sweeping and incorrect claims. Government bonds have very strict terms on repayment and that is for a reason - they need to be exceptionally predictable and reliable to function in their primary role of being reliable bonds.
Trying to postpone or alter terms of debt would be viewed as limited default, as has happened in Greek case. I.e. perhaps the insurance events would not be triggered, but markets would most certainly dump the bond en masse demolishing its value both in terms of cost and trustworthiness.
Finally there is an issue of financing. US needs to finance its government and it needs to finance its private sector. As of writing this a large portion of this financing comes from China for a very simple reason - China gets a lot of money from exports. That money has to go somewhere. As a result, it's investing everywhere, including US debt, both public and private. Sudden cessation of this investment would be a massive shock to the system, and while it would likely not be as fatal as default event on bonds, it would be a severe shock to the system for two reasons: one is the direct need to finance the portion that used to be financed by Chinese, and other in the fact that bonds would have to have increased yields to sell both due to less investors as well as the fact that one of the biggest if not the biggest current investor in the world suddenly ceasing its investments and shifting them elsewhere would have many analysts consider actual downgrading and possible limited dumping of the bond.
All of above events would cause severe harm to US, and by extension world economy, which is why they are unlikely to occur. We are effectively in a state of financial MAD in credit system.