It's not so much messes. Rentals, including short-term ones, are a thing today. But rather there's a lot of value to many people to have the car that they want when they want it, leaving certain belongings in the car, having the car equipped for their purposes whether child seats or roof racks, etc.

While I honestly don't expect door to door self driving in most places for decades, a lot of people who don't use cars much probably underestimate the degree to which many people customize their vehicles. And even if they sit a lot of the time, most of the cost is often in miles, not time.

In a world where car ownership is limited, I can imagine that the car interiors would be changed drastically. Something like a subway car with hard plastic seats and easily sanitized surfaces.