How many of those systems are owned by private people that has no idea what to do about it? Do you plan on suing half the planet?
If you pass that law on Day Zero, I claim that on Day One, manufacturers provide some horribly arcane command-line interface for rooting lightbulbs and washing machines, and add some boilerplate to their shrink-wrap licenses forcing customers to acknowledge that they have admin privileges on their devices.
Problem solved for them, Granny is liable again according to your system.
If that still doesn't solve the problem, the media will take care of it. "Buying this smart lightbulb puts you at risk of being sued for thousands of $$$" can't be good for manufacturers and they'd want to avoid the bad press.
"Selling this insecure IoT-device/phone/router/tv that requires every consumer to become a security expert, and taking no responsibility for OTA patches and so forth, puts you at risk for paying hundreds of millions of dollars in fines and/or damages."