That's true for majority of cases, but a small percentage of Tokyo that needs to visit places taxis and trains don't go is still a large number.

Thinking 10 years down the road, will autonomous rentals be geo-fenced and not suitable for multi-day trips to rural places?

Yeah, so few households that still own a car but don't use for commute choose cheaper ICEs that require no attention during weekdays. Then company cars driven weekdays when necessary remain ICE for familiarity as well as various EV chicken and egg problems.

A car should always be a major consideration for someone's needs. Especially if you have a family with young children or are disabled. Discounting cars as useless because mass transit exists is a terrible idea.