Agreed. There should be a large warning on the console: "Warning to passengers: The Vehicle Systems have been Compromised. Please do not ride in this vehicle. A Service tow truck has been dispatched."
Wow, as someone who grew up "hacking" on my car (i.e., on the engine) this attitude is pretty amazing. What a dangerous, mysterious thing a car must seem to you.
I can see both sides of this one. If I R&R my brakes or repair the steering rack, I can reasonably claim that I did so using good mechanical skills and practices, and prove it in court if necessary. But if I tinker around with a closed-source software system in my car, I literally have no idea what I'm doing. The changes I'm making could affect any and every other system on the car... and nobody but the factory engineers would know for sure.
The best solution is the simplest: no closed-source systems allowed in cars used on public roads. But I wouldn't hold my breath until everyone gets on board with that.
It's different in a number of ways: Suppose it's not you hacking your car, it's an enemy that want you dead. So they disable the brakes. Or perhaps it's possible for an attacker to disable the brakes only when you're braking hard and have a speed above 100 km/h (60 mph).
Or suppose a neighborhood kid is angry at you, have figured out how to hack the system, but haven't yet figured out the difference between "that'll teach them a lesson" and "this might actually kill them".
Or, hypothetically, if system hacks don't require a physical connection, it's wide open for anyone anywhere in the world to replicate something like the file encryption extortion scam[1]: Break into as many cars as you can. Send them a mail saying that you hacked their car. They can take the chance of figuring out what you did on their own, or pay you money to revert it. The scam might work just as well for cars you didn't break into, as long as the owners believe it's a credible threat.
The point isn't necessarily that these scenarios are more likely than in the physical world. The point is that many people have a fair idea how the physical world works, while they have only vague notions about "hacking" in the virtual world. We know that there are new threats, but we don't yet know what they are, so these new threats will be inherently scarier than the threats we already know about. (The devil you know, etc.)
Cutting the brake lines or using a kitchen knife (or a gun bought off Craigslist in the US) to kill the person you hate works just as well and is much simpler.
I personally would like to see various "hacks" adjusting the suspension, brakes, spark timings and other things for a better ride in certain conditions (racing, drifting, mountain roads, etc).
Do you think the same thing about a car where the owner has, say, installed aftermarket brake calipers and suspension components? ... Because that's incredibly common.
Also, the car should not move.