If they crack the OS of one or more automated vehicle models or brands, they can mass hack because the OS's baselines and avoid needing to hit each car separately.Not really.
Plenty of hackers have said hacking cars are not worth it since no two are exactly the same and they have to do one at a time.
They also as with all IPs change over time so ig you don't use it, you lose it kinda thing.
Basically there is more to it then simple hacking.
Hackers see no use in hacking cars since holding people hostage isn't easy in anything not electric basically
And my fear is not about taking hostages, it's about using the vehicles as weapons or just causing mass casualty events.
Like, you think the CCP, DPRK, or Iran would bat an eye at using methods like that to hit the US populace, because I don't.