>- IoT sensors are a thing, whether the "average person" needs them or not. Think remote weather stations, car counting cameras, GPS trackers...

Something tells me that your average municipal government or enterprise isn't going to want a hodgepodge fleet of phones as IOT sensors. Most of the applications you describe don't even need to the phone to be reprogrammed. There's a dozen apps that allow your phone to be repurposed as cameras or GPS trackers today, what's holding back their adoption?

>- "Smart thermostats" exist, surely you could just copy whatever they're doing with ease. And let's not limit ourselves to DIY here.

Yeah but how much is this custom hardware going to cost, especially when you don't have economies of scale? You can get a sleek looking smart thermostat for $150-200. Most people will take that over a tangled mess of wires that a DIY solution is going to look like https://en.wikipedia.org/wiki/File:Ahmed_Mohamed_Clock_by_Ir...

>- Every block of flats I've been in here has had an intercom system, some even have video transmission. Sounds like a job for old phone hardware, no?

So you're going to be gluing a phone next to your door? Sounds like a great way to lose an old phone.

>And just to repeat, I don't want regular people to start making these things out of old phones en masse, I want businesses to have that opportunity. You're arguing against a strawman.

No, you're arguing against a strawman. If you read my initial comment you'd see it states in no uncertain terms that I'm skeptical of the argument that it'll meaningfully reduce e-waste, not that there's going to be exactly zero people repurposing their phones.

Why are you assuming the result will be a "hodgepodge" or "tangled mess of wires"? Unless you take it apart, you won't be able to tell if a product runs on a bespoke PCB with SoC, a Raspberry Pi, or an old phone's hardware. Plenty of commercial hardware today is just an Android phone with some custom apps and system modifications, only it's generally a new, expensive, "enterprise-ready" device.

> There's a dozen apps that allow your phone to be repurposed as cameras or GPS trackers today, what's holding back their adoption?

Personally I think it's the lack of control over devices that's hindering these apps. A common modern phone doesn't let you replace the system UI with some purpose-built app, it doesn't let you run without a battery, it doesn't even let you disable all notifications. The result just isn't up to snuff unless the user/device manufacturer has full access to reduce the system to just the parts they need.

> If you read my initial comment you'd see it states in no uncertain terms that I'm skeptical of the argument that it'll meaningfully reduce e-waste, ...

And you haven't provided any meaningful counterargument so far. You still seem to be under the impression that reusing phones means hobbyists "gluing phones" places, but that's far from what I'm advocating.