Behind every bubble is a good idea bursting to get out, and bitcoin kind of looks like a good idea, at least if you squint a bit. A digital currency without borders that governments can’t control and that allows secret online transactions? I’m in. Bitcoin itself? Not so much.

So is a single bitcoin worth $500,000, $5,000, $500 or $0? I’m inclined to say $0, especially if bitcoin’s value depends on it being adopted as a global digital currency to replace dollars. There is no chance whatsoever that bitcoin can displace the dollar,...