There’s already a feature in the iPhone that if you use an activation lock p/w, and someone bungles it enough times, the phone basically becomes a fashionable brick.
You can remote lock or wipe the phone now, but the thief can plug it in to a computer, reset it to default settings, put their sim card in, and use it or sell it. So currently, just your data is protected. What they are talking about with the kill switch is making the hardware unusable.