The US Dollar is based upon the full faith and credit of the American nuclear weapons stockpile.
If you think General Motors is going to tank, you don't want their bonds, nobody does, they are junk. If Google were to issue a corporate bond series, everyone would eat them up like candy though.
Because the USA is the only remaining military superpower in the world, it's fiat currency is going to be #1 in the world. The Eurozone is #2 militarily, and it isn't a coincidence that the Euro currency is also #2. And places like Zimbabwe have jokes for militaries and jokes for currencies. A few years ago someone gave me a 100,000,000 Zimbabwe note as a gag gift.