Accidental and unexpected loss of power can happen for any number of reasons, and frequently does. It's a situation that EVERY driver should be prepared for, and in fact should practice for. I would love to see it as a standard part of driver testing, in fact; come to freeway speed (60+mph) on a closed course and kill engine power. Driver must be able to navigate and stop safely with no power assistance.
I realize this will probably never actually be a part of driver testing, but the ignition switch is far from being the only, or even the most frequent, cause of a loss of power while driving. I firmly believe any driver who wrecks because they weren't able to handle this situation should permanently lose their license and receive a citation for reckless driving, as well as criminal charges if they cause injury to another driver.
"If you can't do that, you shouldn't be driving" could work, the only reason it doesn't is because we enforce the wrong things in our driving laws. Instead of a focus on actually removing unsafe drivers from our roadways (or teaching them how to drive), we focus on speed laws because they're easier to enforce (and make money from). Excessive speed can be unsafe, but there are many situations where exceeding the speed limit is not unsafe, and a few where it may actually be the safest option. At the same time, in most places "following too closely" citations are almost never given, despite that being far more dangerous than speeding while maintaining a safe following distance.