Slippery Slope Indeed....
While I think it's an interesting concept, there's the law of unintended consequences as well as human inanity.
Mouthwash, possibly aftershaves, perfumes... lots of products with alcohol in them. How does the car know that driver is sober if they're the designated driver with a load of drunken friends? The skin sensor tech is problem with gloves and some skin conditions.
Let's not forget that impaired drivers are not always using alcohol... there's assorted legal and illegal pharmaceuticals that are just as bad as alcohol.
The human inanity part is politicos... you trip the sensors, the plod gets sent for your "intending to drive while intoxicated". Your insurance company will be notified. Maybe others.
It might be better to just push on the autonomous cars although I'm not fond of that idea either since they can monitored and abused by "security" and LEA's.