When has "market cap" been a measure of a currency? This is indicative of the logical fallacy so common in these debates, conflating "store of value" with "useful mechanism for transactions."

It's a Ponzi scheme. There's no inherent underlying value, its "market cap" is strictly a function of more dumb money flowing into the system. Occam's razor, this is the most likely explanation for everything in the crypto space; the model makes sense and explains a lot more than "a new financial asset" hokum.