This is the reason I oppose moving to a push-button system. We've already seen at least one person have an uncontrolled acceleration problem and not have a key to turn off. Push-button HAS to include an emergency cutoff switch. Requiring the user to hold in a button for several seconds to stop the engine is not acceptable.
There may be cars that are not like those I've seen with start/stop buttons, but from what I've seen I don't see any problem.
My car has a start/stop button, when driving above 5 or 10 km/h the button does not shut down the engine when pressed, and it is plausible that it could mechanically malfunction at some time. In any of these cases I can force the engine to stop by removing the electronic key from the slot, either by pressing the emergency release button or simply by ripping it off.
Other than keyless systems, all cars I've seen with start/stop buttons need the electronic key to be inserted in some kind of reader, and I would very surprised if those with keyless systems didn't have some simple way to stop the engine in case of emergency.