The comparison you should be drawing is to human driven cars; not bikes. In this case, I do not think that self-driving cars actually increase the threat of malicious hacking. As it is, most new cars are already drive-by-wire meaning that a hack of the car gives the attacker full control already. Modern cars are also already heavily computerized, which makes hacks possible (and which has been successfully demonstrated by researchers [0]).
Granted, being self driving will give a hacker options beyond, say, maxing the throttle and disabling brakes and steering.
> As it is, most new cars are already drive-by-wire meaning that a hack of the car gives the attacker full control already.
I don't think that's fully the case; you can't tell most modern cars to swerve sharply to the left. You can also still turn off the majority of cars by turning the key. (And of course, for manual cars you can put the transmission into neutral.)
Any cars that have "automatic lane holding" could definitely be told to swerve left. Granted, steer-by-wire systems like that aren't quite pervasive yet. I believe electronically controlled throttle or brakes are more common. Locking individual brakes especially could quickly lead to a very bad situation. Also, putting the car in neutral or removing the key is likely not something that is going to occur to most people in the 1st half second or so that all this is happening.
Granted, being self driving will give a hacker options beyond, say, maxing the throttle and disabling brakes and steering.
[0] https://www.ted.com/talks/avi_rubin_all_your_devices_can_be_...