Since 1972, the US government can always pay its debt in its currency with whatever interest rate it promised to pay on it because it creates its currency out of nothing.
Government bonds should be downgraded due to their respective currencies being devalued, and not nearly as much due to a risk of default.

However, some currencies may be worse than others in certain periods, and in such cases buying US government bonds may be a beneficial move as a ForEx bet.
This is not a recommendation or suggestion to either buy, hold, sell or short them now.