Then you argue that the dollar is a pyramid scheme because people only want dollars to buy things that have real value. It's true that people only want dollars to trade them for other stuff, but, again this is not what a pyramid scheme is. All mediums of exchange that have no intrinsic value are only wanted in order to exchange them for other stuff. A pyramid scheme is not that, a pyramid scheme is an investment scheme in which all revenues come from the investors themselves. [2]
Cash cannot be a pyramid scheme for the simple reason that the revenue streams of an investment in cash add up to zero, and therefore it's not something that an investor would consider investing in in order to earn a profit. Therefore cash is not an investment scheme, and something that is not an investment scheme can't be a pyramid scheme.
[1] https://en.wikipedia.org/wiki/Real_versus_nominal_value_(eco... [2] https://en.wikipedia.org/wiki/Pyramid_scheme
Nominal terms amount to how much of your investment in dollars you lost (or gained) while holding dollars, in real terms. Real terms are all that matter.
Cryptocurrencies are also used to trade for other stuff, as is their stated purpose.
They both have the same characteristic that makes one a pyramid scheme, but somehow not the other. They're fundamentally similar assets, but for some reason one is given a special category carved out specifically for it that actually does not exist. When you take away this special status that is only ascribed to fiat currency to differentiate it for the sake of discussions like this, they're interchangeable assets that share all fundamental characteristics. All fiat currencies are speculative assets.
Exploring this might help you understand what I'm saying better: when people buy dogecoin, say, with dollars, they're not actually just trading dollars for dogecoin. They're using dollars as a medium through which to trade their wealth (again, not money, similar to the value vs price thing) which could be their labor, land, other property, intellect they've sold for dollars, for dogecoin. In both instances, they're trading real wealth for something that is valuable because other people want it.
It isn't about the return. It is about the fact that both liquid storage mechanisms, dollars and dogecoin, have a "value" that is only valuable because at some point someone else will wind up with it when it inevitably becomes worth nothing. For both systems, the value is solely that people want it for now, and when they no longer want it, the people left holding it are broke, and the people that traded it to them for real wealth benefit. This is a pyramid scheme.