One of the problems with criticisms of the design of the car systems is that it doesn't fit the mindset of the car engineers, and places a model over the car that actually doesn't exist in computers either.
Last I saw your average PC was just about as open a trainwreck as the cars we are criticising. There are a huge number of separate processors, many interconnection buses, and zero security. A PC typically has a number of high speed buses (SATA for a start) talking to subsystems with their own embedded operating systems. Then there are the slow buses for trivial stuff (USB 1.1 devices) and faster USB for things like WiFi and Bluetooth. Every one of these device controllers has embedded processors, many with subvertable hardware, and known attack vectors.
I don't hear pious moaning that it should be trivial to add firewalls to all the buses inside a PC. Yet it is essentially the same problem. There are hacks that can pwn a hard disk drive (many of which run say three separate ARM processors and a full multitasking OS). Not to mention hacks that can subvert your ethernet controller or WiFi controller to take over your PC. We all know not to plug an unknown USB device into a PC - but I bet that is a rule more observed in the breach. It isn't trivial autorun exploits we have to defend against now.
Yes, car system security is a big deal. But don't pretend that somehow the mainstream computer industry has trod these tracks long ago and it is the car engineers that are dolts. Everything is built to a price, and when there isn't a clear driver for change, change doesn't happen.
The care taken in car system where the issues are understood makes the mainstream computer industry look like a bunch of idiots blindly walking into walls. These are hard real time systems, and they are tested and simulated to clock edge and instruction boundary precision. But like so many stories of security in the history of computing, nobody even thought it was an issue. (Like the Morris worm, when the first message that went out had the point that was along the lines of - "we all knew this was possible, we just didn't think anyone would be stupid enough to do it.")