"3)What if I buy a couple of 50 pound bags of dog food and want to leave them in the trunk while I’m at work? Or if you just keep your bicycle in the back, as I do."

Well for purchasing large items you could do what I and plenty of others who don't own cars do and just get it delivered. The price of getting stuff delivered to your door has been falling for a while to the point where you can get all sorts of things delivered for cheap or free in places that aren't NYC these days. Amazon has some questionable business practices, but now that people are used to the idea of clicking on something online and having it show up in a reasonable amount of time for cheap or free, you can't really put that genie back in the bottle.

And actually regarding your other item about trips for things like food, I'd much rather have food come to my home than eat it in a moving vehicle. These days you can get a ton of things delivered by third party services that handle delivery for places that wouldn't normally do delivery (bite squad, door dash, etc). I imagine they might be able to streamline their business model by utilizing the same autonomous vehicles to the point where delivery on demand becomes even cheaper. At that point I don't really see the advantage of getting in your own self-driving car to make the same trip when the cost difference is pretty negligible (since you're still paying for your own gas and spending time away from the comforts of home).

I could even see something happening where you basically have a mobile automat that doesn't even require a person other than to load it at the store/restaurant/whatever and delivers things to you at home or work or where ever.