UST Issuing Debt in Dollars Can Never Go Bankrupt.
If you look at the balance sheet of the UST as it stands today assets = $0 with liabilities of $18 or so trillion. From a balance sheet standpoint the UST is bankrupt.
However, bankruptcy is defined not as a negative net worth, but the inability to pay current obligations, and that is where the FED comes in monetizing any additional debt the UST issues.
What makes UST valuable is #1 economic growth or lack of, and #2 inflation or lack of.
Companies on the other hand don't have the FED buying their debt (yet) so when the economic prospects fall, so do their bond prices.