There is a kill switch. If you watch the video you'll see that the Charlie and Chris told the driver that he could turn off the car and turn it back on in order to reset everything.

A simple radio kill switch will stop any future instructions from coming in but it won't do anything to get rid of any persistent effects that the hackers have set. How are you then going to turn down the radio? How are you going to tell the brakes to stop bleeding? How will legitimate remote technicians help you to regain control of your cars systems? Turning the car off and then on completely reboots the system to load from firmware. The only way persistent effects will last through that is if manufacturers haven't guarded against remote firmware changes.

It's mildly dangerous but it's far better than rolling the dice by just turning off the radio. Having a way for people to recover from an ongoing attack is necessary but, OEMs need to do a better job of isolating critical systems from this kind of attack to begin with.

whoosh!