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).

Car rental companies already manage this quite handily. Roughly speaking: economy, midsize, full size, SUV, 4WD SUV.

"4WD, Bike rack, Dog area" -> 4WD SUV.

You'll probably have to snap on your own accessories (kayak/ski/bike rack) when the vehicle arrives. Or if you order in advance, you can pay extra to have a specific accessory configuration prepared and set up for you (vehicle drives itself to local servicing point to get configured).

Perhaps a fleet of self-driving cars could reconfigure on-demand.

e.g. Customer orders a self-driving car with 1 infant car seat. A robot bolts-in a modular infant car seat at the depot and sends it off.