It could have been so simple
Entertainment network with USB port to plug in any phone, linked to speakers and everything Internet-facing on its own network. Your in-car webcam has been hacked ? No problem, that's all that is accessible. Just don't shag in the car and you'll be fine (with a bit of black tape on the webcam camera, of course).
Car CAN bus totally seperate, with high-level fob key ciphering to open doors and start engine, and under-the-hood/bonnet USB port for maintenance. Unhackable unless you get physical access (like most non-computer things these days).
Why, why, WHY did they throw everything on the same bus ?
Well, in the end, it doesn't matter. A few hundred deaths and a raft of class-action lawsuits will certainly sort the issue out.
Unfortunately, it seems that that is what it is going to take.