Worth remembering that at least bitcoin is backed by some value of work, what is USD backed by again? Debt :-D
Obviously, I'm being a little bit facetious...
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.
2. USD is backed by the law -- laws concerning debts, laws concerning taxes, laws concerning torts, etc. The law is what makes USD useful, which is why USD has value.