The hope is that autonomous cars will be much cheaper once you take out the human element; when a taxi costs $30 for a routine trip, and Uber/Lyft still have to pay their drivers >minimum-wage, you cannot cater to the elder niche of doing groceries and making regular doctor trips, not to mention the useful aspect of 24/7 coverage.

There's also the psychological issues: elderly people don't want to be a 'burden' or rely on strangers, and autonomous cars neatly solve both issues.