"backed by" is a euphemism for "limited supply" that dates back to 35 dollar ounce days. Bitcoin isn't backed by anything. It is limited by the mathematics of random numbers and clocks.

Neither one is a Ponzi scheme. One of them is a terrible choice, perhaps even a scam. but not a Ponzi.

In order to qualify as a ponzi scheme, the thing has to promise abnormally high yields while either hiding substantial risk or more often the inevitable collapse of the scheme.

You can say pretty much anything negative about USD and be correct, but not that it's a Ponzi. No, it's not a pyramid scheme either.