> if this was a simultaneous saturation across everything that a company could do with their current resources.

That's exactly what I am describing. It's not that a particular style of vehicle is no longer in demand due to a shift in preferences and factories would need to be retooled to create a different type of vehicle, demand for all vehicles has fallen. Yes, the people who once worked on those auto assembly lines will generally go on to do something else, but it won't count as an increase in productivity in car production.

More generally, demand for pretty much all US domestic manufactured goods has flatlined or fallen. Production capacity is not the limiting factor in almost anything mass produced nowadays. Even dramatic reductions in manufacturing cost aren't going to induce any demand. Throw in demographic changes where there are fewer people who have needs to be satisfied, and better products which don't need to be replaced as often, and we ought to expect falling demand across the board.