A Ponzi scheme involves paying out old investors with new investors money. There's a few steps inbetween but Tether (and by extension the whole cryptocurrency market) is very much within that definition. If people stop buying cryptocurrency tomorrow, the entire system collapses, because it's entirely dependent on new money coming in. If you disagree, a breakdown of where you think this differs from a Ponzi scheme would be much appreciated -- is there another type of fraud scheme it fits into in your view?