In push to start cars, holding down the start button is the normal way to turn them off, so it is not unreasonable to attempt to do so for an emergency shutdown.
Having said that, I would like an emergency stop button in cars (preferably one that works indepenently of the onboard computer).
It's certainly a reasonable thing to try, but as as your aunt eropple notes, an emergency shutoff should be something so obvious that a person in fear for his life would think of it.
Such as an ignition switch which you turn to the left to physically cut power to the ignition system, like we used to have back when cars weren't so overcomplex.
Having said that, I would like an emergency stop button in cars (preferably one that works indepenently of the onboard computer).