I think the magic sauce is utilization. A fully autonomous car's utilization rate can be close to 100%.
Higher utilization --> same demand for mobility hours/miles can be satisfied with fewer vehicles --> ceteris paribus, fewer vehicles sold every year.
Viewed through this lens, it seems inevitable that, in equilibrium (ie once the non-autonomous to autonomous fleet transition is complete) annual per-capita demand for vehicles will be lower with the advent of fully autonomous cars.
