True steer by wire systems are still pretty rare. There's like 3 or 4 models of brand new cars with them. For a couple years the Infinity Q50 had a clutch in the steering column that would allow for steer by wire or manual steering, but the electronic steering was pretty universally hated so they took it out.
Same with brake by wire. I won't say there are none in the world, but getting the feedback correct has proven to be really difficult, so pretty much all systems still have a physical connection to the hydraulic piston.
All the new lane change, auto drive, auto stop systems can pretty easily be over powered by actual hands and feet on the controls still. For the time being, hacking a car to crash and kill someone would require them to be very passive about the whole experience.