Sure, and what happens then? Nobody buys new gov bonds; non-gov debt rating stops making sense. For the financial system it's a shock so huge it's better to only do this if no other solution is possible, at which point the gov is bankrupt anyway.
Sanctions, black-listing and potentially more for anyone trying to play cute and do anything that "disadvantages the US". The rest of the world is forced to comply in enforcing it all, because nobody has backing power to stand against the US in any way.
And anyway, most US bonds are held by US domestic entities.
From across the pond, the USA looks like a has been who relies a lot on old merits. The world got bombed to pieces 80 years ago, but has step by step rebuilt itself.
The advantage the US had has been squandered by ineffective leadership.
Forget those, how well are the vaunted US sanctions working against Iran?
Now they're running low on ammo, without having accomplished anything of strategic value...
You mean like the compliance of weak, sanctioned state of Iran with US demands?