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).
I don't think it works like that. Manufacturers stop providing OS updates as soon as they can because providing any kind of support has a cost. Planned obsolescence means "they care about making it obsolete" (active). But the reality is that they just "don't care about keeping the product alive" (passive). And the only way to make them provide updates is to force them by law.
> 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
You can relock the bootloader with the FairPhone. You will still have a message saying it's a custom OS, but I don't think it compromises the secure boot, does it?
> many apps simply refuse to work with this setup
I heard that there are apps that refuse to work with an unlocked bootloader, but I haven't heard of apps refusing to work with a relocked bootloader. Is that a thing?
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.
But hypothetically, if we were to want to survive, such regulations would be some of the very easy steps to take (and by far not enough, of course).
And again, I think you're right: it's far more likely that we as a society will just collapse, so maybe it's not even worth wondering what we would do if we didn't want it.
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.