I agree any real car hack will likely not end well. This solution also introduces the possibility of a new vector for state sponsored attacks, which should be concerning to some.

The utilitarian decision is still to upgrade everyone, because the people who would hack cars to kill would likely do it another way. This may become the new improved way to "cut brakes." A state mandated system could upgrade hacking charges to serious felonies, which deters most except state-sponsored and psychopaths. I have a feeling that real car hacks will be as rare as poison Halloween candy

Also in a situation where someone is hacked, having a dense recorded model of all the roads in a city would be very handy for investigation, since the hacker would likely have to be near the car.