So in theory can normal cars. Many taxi companies (and I'm assuming at least some Uber drivers) share one car between 2-5 drivers to maximize the time on the road.

The other aspect you're not considering is that even after you take economies of scale into account a driverless car will almost certainly cost more than the market price of the current median Uber car. I'm guessing that $3000-5000 will get you a perfectly Uber-worthy car that will give you years of fairly trouble free use. What will a driverless car cost?

And that is ignoring the fact that most Uber drivers would have owned and maintained a car even if they had had a different job, so the only 'cost' is the marginal cost of extra maintenance being used as an Uber car causes.

Also I wouldn't be surprised if maintenance costs will also be higher at least in the short-to-medium term due the complicated and experimental nature of the components involved.

So even if there are long term savings I imagine that they will take a long time to materialize and even longer to show up in fare-prices, unless Uber decides to subsidize driverless fare to kick start the market.