That's already possible, it's just expensive. Remember when the kindles[1] used to have the option for built-in 2G/3G connectivity "for life" to download books from Amazon that you never had to sign up for or maintain or pay for. Until networks dropped support for 2G and 3G in 2021.
Exactly, like Amazon sidewalk. They market it as an easy configuration feature but it can also be used for tracking and telemetry. So your smart TV can call home even when you don't connect your own WiFi.
Arguably all your IoT devices already do. There were multiple startups advertising "routing at the edge" or some other such bullshit maybe ~5 years ago. I have no idea what happened with those ventures but these days multiple ISPs offer the general public roaming WiFi access via the APs of their customers so I think it's safe to say that ship has long since sailed.
Come to think of it how cheap are LoRaWAN radios these days? That's another option.
I'm sure this costs money to use, just like regular cell service would for the same theoretical IoT device. That's probably the main barrier to having its own network.
Phoning home with a few packets here and there via the cell network is quite cheap. An adversary, pardon I mean OEM, doesn't need to upload 4k video to gain value here.
If anyone walks past your house with a device in the same bluetooth p2p network as your device this is already true (Amazon sidewalk). This could give them a more uninterrupted connection though.
Stucco itself is not conductive, but it's usually applied on a base of metal mesh (similar to chicken wire, but apparently actual chicken wire isn't appropriate). Some projects use fiberglass mesh as the lath, and some may use traditional wood lath, but my understanding is the majority of stucco for house construction uses metal wire mesh for the lath.
Ps in order to be an effective Faraday cage it must also be grounded and no gaps in the metal may be larger than the wavelength that needs to be blocked. Which is only a couple centimetres (or around an inch or so) for the highest ones
Stucco mesh needs to have no gaps for its structural needs, it's commonly overlapped by 3-6 inches. And, depending on the surface, the mesh may embed into the ground, offering a ground connection (if poor)...
The mesh does have many openings, but they are around the size you mention, so it should be ok-ish.
Otoh, there's like doors and windows. I believe a partial faraday cage still significantly reduces signal strength though. It'a difficult to use cell networks in my parents' stucco house, but step outside and it's fine. But stucco is also very similar to concrete, so that could be it, too.
