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.

[0] https://www.ted.com/talks/avi_rubin_all_your_devices_can_be_...