This is not completely correct: your Leaf does have a purely electronic throttle and the e-brake is actuated by a solenoid, but no production passenger car presently uses brake-by-wire, and the first (and presently only) production passenger car with drive-by-wire is the Infiniti Q50. While your Leaf does have additional electronic braking means to make the regenerative system work, if the ECU lost its magic smoke the service brakes would still work. As to drive-by-wire (perhaps you're confused with electronic assist?), on the Q50 at least there is a physical linkage to the steering in case of electronic failure.
Thanks for the correction. I ass-u-med (and actually do recall reading it somewhere, but just some dude on a Leaf forum probably) that because of all the computerized fiddling required to balance regen braking with mechanical braking, the brake pedal was just a fancy potentiometer.
As for steering, I went to garage between builds and looked: yup, there's a mechanical linkage in there. Shows how often I lift the hood on that thing. :-)
I'll leave my error-filled post as-is as an example of how being a pro mechanic 20 years ago doesn't mean you know everything about modern cars. :-P
Electric cars potentially have additional braking redundancy. Even if the brakes failed entirely, they could drive the motor in reverse to stop the car. I don't think any of them actually do this, since total brake failure doesn't happen very often, but they could.
You pretty much have to trust the ECU. A failing ECU could just punch you in the face with the airbag and floor the accelerator. Fortunately, they seem to be pretty reliable.
Your examples of tire/ball joint failure is of course much better.
I realize it's just one example to make a point, but automotive master cylinders don't completely fail all at once since about fifty years ago. What causes a master cylinder to fail is a failure of the seals. But because modern master cylinders have "dual channels" (for lack of a better description), half of it might fail, but it's so unlikely for both sides to fail at the same time I never even heard of it in my years as a professional mechanic, let alone actually seen such a failure. Though a self-driving car would still need to account for the fact that only half the wheels have functioning brakes.
Any other failures I can think of on modern cars are going to be so much of a failure that merely getting the car to the side of the road might be a challenge, let alone worrying about the bus full of nuns and orphans in front of you. Examples would be catastrophic tire or suspension failure (ball joint breaks loose, for example, making the car very difficult to steer).
On the flip side, it could be argued that late-model cars have worse failure modes. By that I use our Nissan Leaf as an example. There is no direct mechanical linkage on any part of the car that makes it stop, go, or steer. Parking brake is electrically actuated. Brakes are run on a wire. "Gas" pedal is wire. I'm pretty sure the steering is drive-by-wire, but I'd have to go back and look it up to make the claim with confidence. I'm sure there are redundancies, but I'm picturing an ECU letting its smoke out and then you might as well just let go of the wheel and pray to your preferred deity.
Anyway, this has been Captain Pedantic checking in, thanks for reading. I now return you to the point you actually trying to make.