Is utilization over time important? If we assume that people will still travel the same number of miles[1], then while there will be fewer cars on the road at any given moment, they will wear out more quickly as they are utilized more heavily.
Another way of thinking of it is that there is demand for a certain number of vehicle-miles per year. In the aggregate over the long term, it is no different if we buy 5 cars, each of which lasts 5 years, or if we buy one car every year for 5 years as the old one wears out[2].
Higher utilization should make fleet replacement a smoother function over time (whereas now, you can see dips and spikes as people decide when to put capital in their cars based on factors like the overall health of the economy).
[1] It is not clear what the impact of driverless cars would be on mileage. It would eliminate some useless trips (e.g., doubled miles for dropping somebody off). But the convenience may mean that people simply use it more.
[2] I'm assuming that wear and replacement time are purely a function of miles traveled. This is not entirely true, as age is a factor, but I think mileage tends to dominate in the average car (i.e., one that is being used, not one having its belts dry-rot over many years of neglect). Companies that maintain car fleets probably have data on this.