I can't help but quote this again: «The U.S. has a comparative advantage as a safe financial haven. We have the world’s strongest military and a stable government. Thus, we have a comparative advantage in producing debt, primarily government debt. That is not necessarily a good comparative advantage to have.» (Arnold Kling)

If there was time to downgrade US debt it would likely be the time when the US rapidly moved and even outsourced its production to China, shutting down local factories. That was the time when the risk for the US might was increasing, should the US-China relations sour (as they have). But of course downgrading wasn't happening then; I suspect it was the other way around.