With regard to them being useless: that argument falls apart very quickly when challenged. Unless you actually believe that software agents won't cause a massive economic shift, and that a weaponized financial system is really a good thing that brings stability. As far as it being a "ponzi scheme", a pump and dump, a tulip mania, a beanie-baby fad, etc... Just look at the time scale for a big clue as to why such an argument is laughable.
To look at why this counter argument is laughable you simply have to do a bit of research into actual ponzi schemes:
* Bernie Madoff ran a ponzi scheme for ~18-28 years
* Reed Slatkin, 15 years
* Dona Branca, 14 years
* Tenka Ikka no Kai, 14 years
It's difficult to actually classify crypto, but it certainly falls into greater fool theory (https://en.wikipedia.org/wiki/Greater_fool_theory).
The larger point being that any plausible definition for fraud applied to crypto actually undermines the legitimacy of the modern financial system to a much greater degree.
