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.
To facilitate planned obsolescence, manufacturers stop providing OS updates after a relatively short time. And then they cease providing security patches after a... still relatively short time.
If you unlock the device and install a custom ROM, which may or may not function adequately for you to begin with, then you're probably also compromising secure boot, which is a problem for the security model of how many people use phones -- and many apps simply refuse to work with this setup (whereas the obsolete OS with no security patches is considered fine, apparently).
IMO we should put in the law that manufacturers have to mainstream their device and provide a way to flash an updated firmware. There is no way they do it without being forced, because it's a pure source of cost for them.
That's how it works: companies optimise in the legal framework we give them. Regulations set that framework.
There's a reason Pine64's devices (which are made out of parts with available public datasheets as much as possible - they don't do the software side of things) are mostly made with parts from a few generations ago, whose manufacturer doesn't care much any more.