What's that got to do with anything? Why should driverless cars be expected to last as many years as current cars? If you're only paying for 20% of the car's lifetime cost, then you're not going to mind if it has to be replaced five times more often. In fact, you'll probably like it - that's why anybody ever leases cars.

It's quite reasonable to expect that it will be economical to pay something like 20% more for the drivetrain to make it more durable, but that's not a radical re-design.