Mercedes-Benz unveiled its dream of a fully autonomous multipurpose vehicle this week. The announcement was full of buzzwords—the modular Vision Urbanetic “enables on-demand, sustainable and efficient movement of people and goods” and “reduces traffic flows, relieves inner-city infrastructures and contributes to an improved quality of urban life.”

Hardly a week goes by without fresh signposts that our self-driving future is just around the corner. Only it’s probably not. It will likely take decades to come to fruition. (Even...