You might want to look around a bit, your assumption is incorrect: there's a surprising amount of appliances that are not IoT, but do rely on such unexpected side channels such as AC-as-a-clock - not everything has a network port, a quartz oscillator or even a GPS receiver, but most appliances do have a power plug.
Again, you are assuming that everything has enough power to run a networking stack, or even that there is Turing-equivalent hardware to run anything at all. (Yes, yes, I know this is HN, where any device is fit for a computer; most of the world isn't.)
My water heater from late 1990s has a cleverly wired set of transistors - not even chips, the logic is literally wired into the product: cheap yet resilient. Connecting that to the internet would need adding some sort of computing device, adding several orders of magnitude in complexity (you can ping it and now it's also brittle, yay).
(I assume that you mean "to the local network, which is then connected via more conventional means, oh, and did I mention you need yet another computer to route data to the remote end?" - from what I've seen, attempts at WAN-over-power have largely been abandoned, even industrial IoT went for LoRa and friends. And from what I've experienced, LAN-over-power is a last-resort desperate hack: better than nothing at all, but barely better than two cans on a string w/r/t packet loss.)
I'm not assuming anything, I'm not participating in this discussion (and I agree with you). I'm just sharing an interesting information for the GP because it's not well known. You're right that it's not really usable in production; but it's nice for some simple home automation.
Sure, it's called "Powerline networking", you can find cheap modems that you connect to your internet connection and then you can use other modems as clients.
