To me, it’s a hugely frivolous issue. U.S. Treasuries receive a credit rating every day. It’s better known as the price. I don’t see how some rating from a questionable firm could possibly have an impact greater than price movements. If anything, a downgrade would merely confirm what the market is already saying.
There’s also the fact that the government owns the printing press, so they’ll get those dollars to bondholders one way or another.
I would go as far as saying it’s safe to look past all forms of debt measurements and ratios, and concentrate on Treasury yields. Is the debt too high? Depends, what are T-bonds yielding? If their yields are going up, then yes. If not, then I’m not concerned.
There are lots of good reasons for lower debt. But long-term nominal GDP growth can easily top current yields.