For a shared car that you can summon, any road-mile it spends between a drop-off and a pick-up is a new road-mile created by swapping 2 parked cars with 1 moving shared car. These have to exist by necessity as well since trip demands are very uneven.

I wouldn't bank on ride-sharing either. Only a fraction of ride-calls are shared, and ride-calls are of course on a fraction of road traffic compared to private vehicles. Ride-sharing is probably going to be less popular in autonomous cars since you don't have the driver to act as a buffer with whatever weird stranger you can end up with, and the financial pressure to share a car is lower when you have no driver to pay for.