Earlier quoted context omitted.
First, the word of the powerful is worthless, because power corrupts. People might keep their word because they're honest, or because they're accountable, but the "most powerful organization in the history of civilization" is by definition not accountable, and being an organization, neither is it honest. Second, the US government doesn't give its word that dollars are usable for anything other than paying dollar-deno…
That's just the tin foil talking. Look at it this way if you must: the rich and the powerful, in the US, have vast amounts of wealth stored in USD and USD-demoninated instruments. It is therefore in their best interests to preserve the value of the USD, or at times slightly inflate it. The risk of your second paragraph cuts both ways. If the euro collapses tomorrow, you might need only US$13 to pay that debt.
You're suggesting that US elites are systematically long dollars and have the power to control the value of the currency, including overcoming coordination problems. If that were true, we'd be seeing dollar deflation. Instead, we've seen about eight decades of uninterrupted inflation. This means one or both of your premises are wrong. I suspect both.
As for the "risk", I didn't mean to suggest that USD is particularly risky, certainly not more risky than EUR. I was just pointing out that the USG isn't "backing" USD even in the sense of Bretton Woods.