Paul Krugman is irritated now because the cost to insure against US default has increased.

He and some Taleb think that those contracts would not be honoured anyway. The contracts are more robust than they know. Taleb should have known because he is a former trader. I didn't expect the same from Krugman because he is not an expert or capable of using Google to find background information for his columns. Or maybe Krugman was in propaganda mode for nothing. Those who buy and sell CDS ignore him.

In case of global thermonuclear war, some of these contracts might not be honoured. In some contracts the insurance payment has been paid upfront :-), thus a global thermonuclear war is a problem of the seller of the insurance.

The Federal USA has defaulted twice earlier. Excluding some catastrophes like the eruption of Yellowstone or thermonuclear war, there are still nasty economic conditions in which the better alternative is to default instead of causing (hyper)inflation.

See some of the comments in the link for more information.