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.
If you upgrade a phone to get a new one with a better camera, well, the processor on the old one is probably decent still, it could be a mini PC where the camera quality doesn't matter.
Also, it's a status symbol, you can't just _not_ upgrade.
This is a huge part of the change we need. I felt proud in a way to show off that I was still using an iPhone 8 until a couple of years ago, and I admire some (techy) people I know still using a phone from that time.
Is pride a healthy, wholesome motivator? May be not, but we're human.