It's an old chestnut that buying a salesman's car with lots of miles is a lot better than buying the same vehicle from a housewife that only used it to go shopping and shuttle the kids to school.
Besides, who says that the driverless car isn't more like industrial equipment instead of the current consumer crap, made to go on "forever" to justify its high price?
While designing cars to last longer would increase the weight of some components, for example allowing more metal around the cylinders so that they can be re-bored a number of times, this could be at least partially by having a much reduced burden of crash resistance.
Some things could be alleviated simply by avoiding the maintenance / cost cutting shortcuts used today. For example fitting grease nipples to ball joints and greasing them regularly makes them last a great deal longer than the sealed units which are riveted to swing arms today!
You are missing the obvious: the cars could just as easily go electric and the severe reduction of moving components will already increase their service life dramatically (sans battery replacements depending on how battery technology evolves).
Besides, who says that the driverless car isn't more like industrial equipment instead of the current consumer crap, made to go on "forever" to justify its high price?