My conclusion is that car sharing services are quite expensive. I can do no more than 3 or 4 short rentals a month before the cost start to resemble that of owning a used car. I do save money, but only because I drive a tiny fraction of what I drove when I did own a car.
Of course, car sharing is not as efficient as SDCs that can move around and maximize usage, but even SDCs would mostly end up parked at night and off-peak hours. Coupled with your point #4, I'm afraid it won't be long before driving the current average yearly mileage using those services become massively, prohibitively expensive.
I'm not sure how that would be a gain for anyone but the environment.
If I was using an on-call car service that I could ping 10 minutes before I'm ready to go each day, I can do that without triggering my concerns.
*Note however: I currently commute by bus since I moved to Seattle, and I make up for hating being ready by an exact time by instead choosing a bus route that has a bus every 15 minutes.
2. User feedback could be a useful deterrent against messing up someone else's SDC, in the same way that both drivers and passengers can leave feedback with Uber today.
3. True, but one can adapt to this.
Or maybe i'm just hopelessly optimistic... :)