There's a more fundamental problem with the idea of market cap. Bitcoin (and other assets) are worth whatever value people attach to them. The current market price is only a measure of what the marginal value of Bitcoin is; i.e. it's worth more than that price to the last buyer and less to the seller.

Since many of us are not even remotely tempted to sell our Bitcoin at these prices, it's easy enough to state with confidence that the Bitcoin we hold are worth far above the market price to us.

On the other hand, most people still wouldn't buy Bitcoin at any price. It's worth zero to them.