Tesla CEO Elon Musk was in his element Thursday night, announcing to grand applause that the company will build a truck that will change transportation and the world’s fastest production car. Whether Tesla can actually produce and make money on its mass-market Model 3 is a less exciting but far more important topic.

In unveiling a semitrailer truck and new version of Tesla’s Roadster sports car to a crowd of delighted fans, Mr. Musk sold a bright future. Each new product, at first blush, offers highly compelling features....