But the fleet is heavily overprovisioned now. We're just used to absorbing the costs of the overprovisioning. But the vast majority of Uber rides (or of passenger vehicle travel generally, really) don't require five seats, don't require trunk space, etc. We just equip cars standard with these configurations for the small fraction of trips that actually require them. In some sense self-driving cars should actually reduce overprovisioning by letting single-occupant trips use smaller vehicles (and hey, in a pinch, cars with bike racks on them work just as well carrying people without bikes, just like five-person cars can carry a single person alright).