This ^ Exactly this. Now if every person in a rural area sold their car back to the manufacturer and then those cars were used to service those rural areas than there is a possibility. But cars will still be purchased because of rural areas. I live 120 miles from my job. My neighbor is a mile down from me. If I want to drive to nearest town, where a car service would most likely be located, I'm not waiting the 30 minutes for it to pick me up. And it doesn't make financial sense for them to have one closer to my house.

If you don't drive, waiting 30 minutes for a robotaxi to show up is vastly better than having to rely on someone else to ferry you around or being stuck at home.

While obviously density improves the efficiency of almost any transportation system I don't understand why driverless vehicles would ever be less efficient than traditional cars. Sure they will initially cost more due to additional sensors and computers there will also be saving from removing unneeded human controls. Lastly accident/fatality rates are several times higher in rural areas so the potential safety gains are larger.

What's that economic phenomenon where a decrease in the cost of something results in an increase in the overall amount spent on it?

Once they are safe and reliable, autonomous vehicles will quickly replace the subsidized demand response bus services that run in rural areas.

(drivers are a big expense for such services)