They aren't Ponzi schemes either, nor are they a type of fraud. Their continued rise in value may rely on "Bigger Fool Theory", but that is true for many other investments that aren't Ponzi schemes either.
A Ponzi scheme doesn't require "bigger fools" or really any sort of fool. Many Ponzi schemes start out as honest investment offerings and only turn into fraud when the fund manager starts hiding their losses.
IDK, literal definition of Ponzi or pyramid says it does require "bigger fools" plain and simple.
You seem to be saying that the "too good to be true fund manager is hiding a Ponzi" doesn't require fools, because the Ponzi is well hidden. Hidden Ponzi is still a Ponzi, hiding it is a (another) financial crime; and "too good to be true" is still magnet to fools. The last customers will still eventually take the fall, that's the structure of a Ponzi whether they know it or not.
None of this is relevant; You're talking about how not all quadrupeds are horses, the issue is if all these horses are quadrupeds. These coins continued rise in value (without anything underlying) while relying on "Bigger Fool Theory" is an example of a Ponzi and a fraud.
What distinguishes a Ponzi scheme is that it promises returns that are paid out with money from new investors. What makes it a fraud is that the source of these returns is misrepresented. It is mathematically doomed to fail eventually.
A commodity like cryptocurrency (or precious metal) doesn't have returns. It may appreciate, depreciate or maintain its value. If the only argument for its appreciation is that "some bigger fool than me will find it to be more valuable", then that still does not make it a Ponzi scheme, nor does it make it a fraud.
A Ponzi scheme doesn't require "bigger fools" or really any sort of fool. Many Ponzi schemes start out as honest investment offerings and only turn into fraud when the fund manager starts hiding their losses.