If we don't generalize anymore on national terms (getting tired of the US/China divide, China banks just hold 1Tn I thought), but think in terms of banks, financial institutions, private investors and companies: would the interest even go up that much?
Most institutions/small time investors can find other places to go with their money, but does the same hold true for banks or big funds?
I mean, are all the international banks not just that much tied up in US debt that they have no other option than to (re-)buy US debt with their dollars, no matter what? Or even, does an informal agreement exist between all these banks that it is just better to roll over debt of the US because of the consequences if they don't? They have a lot to lose if the US economy stalls, and suppose one of them owns several hundreds of millions of US debt, what could it do with it? Buy gold? They would suddenly need to find another market which would be big enough and deliver the same yield, and they would need to do that fast - money just standing around loses value.
Maybe losing AAA doesn't mean anything.
If the amount of money 'fighting' to be reinvested hardly changes, the interest rates can't change. If the amount of money, next time US debt is rolled over, suddenly dramatically changes -even if they strike a deal, but banks find it now necessary to reposition- I guess interesting times are ahead anyway.