Yours is a paranoid theory. This future you're worried about is just as possible with elevators and yet we don't see it happening. It'll be just as difficult to get away with because of how unusual accidents will be with self driving cars.
Elevator accidents are rare (hence: suspicious), people transit them for brief periods of time, usually in the company of others, and it's difficult to known when your intended target is in the elevator to attack them. If you're going to target someone via an elevator attack, you've likely got far more effective and specific vectors which would work.
Cars tend to be assigned to specific individuals (permanently, on an ongoing occasional basis for car-share programs, or for a trip duration in the case of dispatched livery) and, well, accidents happen, even with automation (other vehicles, mechanical failures, road or environmental conditions). Moreover, if your intent is to convey someone somewhere, it's a lot easier to do this via an automobile (which can travel anywhere on a paved road and non inconsiderable options on unpaved roads) than in an elevator, which, with the exception of Mr. Wonka's design, tend to follow a rather predictable and limited course.
If I were in charge of threat assessment for a VIP/HNWI, I'd very much take this threat into consideration.
The difference, I think, is that the majority of elevators today rely on mechanical systems for control. Sabotaging a mechanical system in secret is a lot harder than sabotaging a computer-controlled system that is networked to the rest of the world.
