If they can be used like that, why couldn't they be used... as phones?
Changing phone every two years is not sustainable, even if the old phone is used as an IoT wall terminal: it's still "consuming" one phone every two years. In a sense, an old phone in a drawer uses less energy than an old phone staying powered to control a lightbulb.
> planned obsolescence
Nitpick: I like to call it "premature obsolescence". Planned obsolescence is the idea of engineering the product to not last more than some time. I think nowadays it's often not the case; rather we engineer the product to last for the time of the warranty (1-2 years) and not more. And a product dying after 1 year is "premature", even though it was not actively engineered for that.
This isn't hard. And it saves a ton of money.