The new Roadster has questionable commercial value, but as an attention-generating product demo it has already worked spectacularly. Elon Musk unveiled the long-delayed Tesla Roadster this week by having it accelerate to 60 mph in under a second, cling upside down to a ramp and kickflip over Tesla’s existing vehicle lineup using SpaceX-derived cold-gas thrusters. Then social media did exactly what Musk was countin’ on it to do. Fans circulated the footage because it looked impossible. Critics...