Can the mechanism break? Of course. But so can your wireless key. So can any one of dozens of computers or sensors required to run your engine. Nothing new here.
The nanny state argument was old when people were protesting seat belts.
You’re thirsty, so you pick up your favorite—-a nice can of Coke. Suddenly a chime from the dash and the engine power drops. ‘DRUNK DRIVING DETECTED” flashes as the car pulls itself over to the side of the road and cuts the engine. A swirl of snow passes over the hood. You push the start button again, but now “INTOXICATED DRIVER DETECTED. WAIT 8 HOURS BEFORE STARTING” appears on the screen. The chill from outside rapidly permeates the cabin as “NO SERVICE” glares ominously from your phone. It’s only another 9 miles to town… maybe you can make it…
Alcohol detection from your breath isn't some new, untested thing, it's been around for over half a century. And it's going to mistake a coke for a beer. Or rather, it's as likely to make that mistake as your exhaust sensor is to break, rendering your car equally "dead".
I think the proper way to encourage seat belt use would have been to allow insurance companies to not cover injuries to insured motorists who were found to not be wearing seat belts at the time of the accident.
Ultimately that just means we as a society pay for it, since they're still going to get emergency treatment but won't be able to pay the bill.
IMO any of these approaches are unlikely to do much, because most people don't consider the potential consequences when they're assuming something won't happen to begin with. Or IE. If people aren't wearing seat belts, they're already assuming they won't get in a crash, so the potential financial consequences don't matter. If they were actually considering such things they would probably be wearing their seat belt, since the potential risk of death or being maimed is always there regardless of your insurance policy ;)
Also FWIW I'm pretty sure the police cannot pull you over for not wearing your seat belt, they have to be pulling you over for some other reason (Not saying it's not still a problem, but there's some nuance there).
Accepting a philosophy of withholding medical care in cases like that would lead to a society of cripples and dead. I don't see how that's desirable or an improvement.
Denying health insurance would just ensure medical bankruptcy and financial woes for anyone who was believed to have been seatbeltless, regardless of proof, since this isn't a court of law, but a "private company" decision.
The backup key is for opening the door.