That's not exactly true. We benchmark debt off of a theoretical "risk free" rate of return, not US bonds. We tend to use US debt as a proxy for the risk free r

That's not exactly true. We benchmark debt off of a theoretical "risk free" rate of return, not US bonds. We tend to use US debt as a proxy for the risk free rate, which doesn't exist in real life. If US debt were no longer perceived as approximately risk free, we'd just stop using it as a proxy, and come up with some better (say, US treasuries minus half a point).