Sam, I suspect your analysis is correct but the ultimate conclusion less so. The problem with shared-use vehicles is the problem of “the tragedy of the commons.” Who wants to summon an AV for one’s journey home only to find the previous occupant(s) left stale food on the seat, a bad smell in the air, and cigarette butts on the floor? As the AV belongs to no single occupant, a significant percentage of occupants will therefore treat it poorly, leading to overall low satisfaction scores among most occupants.
Hence despite the economic rational for shared-use vehicles the psychological component of human behavior is likely to predominate. We humans understand very little about economics (which is why, in general, we’re such poor economic actors) but we understand a great deal about how “our” car represents us in the world and is our miniature mobile cave within which we restore our fragile egos.
So if AVs are going to be as expensive as you project, the future doesn’t look good for AVs as a large-scale phenomenon.