Not sure I agree. Many of the important mileage and safety features these days in cars (traction control, direct fuel injection, hybrid tech, etc.) are only possible / beneficial through the use of very fast DSPs under the hood. I agree that there are other features in cars that have less to do with basic functionality and more to do with bling (watch movies on the dashboard?) but I have to say that given a choice between a car with traction control and one without, I'd pick the latter, every time. Saved my bacon several times, and CPUs make it possible.
I wouldn't be surprised if at some point we may see car-bus systems extend out to the periphery, i.e. turning light assemblies, etc. being powered by a common voltage bus but commanded via a CAN or similar RS485 communication system to save on wiring and weight.