The circulation of bank notes by far exeeds its backing, so it is enterily impossible for the US to fullfill its side of the promise.
Technically, the US hasn't promised convertibility for a long time. I'm pretty sure the same is true for most of Europe, and most major modern economies. So, no need to single out the USA, there. You can pull the same trick with most "1st World" countries.
This leads back to the topic: Who, in his right mind, would actually try to aquire a pill of unfullfillable promises with no chance to ever make money from it?
The promise cannot be immediately fulfilled, no. You've no chance to get rich quick on it. But US bonds are still today considered a solid conservative long-term investment.
Let's assume you're a villain with oodles of cash. Or, more plausibly, you're a villain that can make a whole mess of banks believe that you're actually a whole lot of people with good credit, so that you take several trillions out in loans, and with those you buy up the US National debt.
(Never mind for the moment that large amounts of that paper is in the hands of folk that are sitting on it as long-term investment - you cannot buy what is not being sold.)
But say you have done this. You then default on all those loans, and the banks get your assets. The banks still have value (those assets are still considered solid in the long term), but they've just lost liquidity, massively. We've recently seen what happens in that sort of situation. Banks stop loaning out money, and you get something between a recession and a depression.
If you, as a villain, have something to gain from economic depression - say, you want to rabble-rouse and take over countries - you have now set the stage for your real goals.