I know this sounds pithy (and it is!) but you'd be surprised exactly how cheap and cost effective Wi-fi enabled SoCs are. A lot of the time we're getting Wi-Fi for free, and most of those SoC's don't have the Ethernet controller by default, so it's more cost-effective to use Wi-fi if it can fit your use-case.
Other physical protocols/connection types can be supported of course (I wonder what the longest I2C run ever is), but when you're talking about a retro-fitted client like this is, Wifi or wireless protocols in general are best.
Purely from a materials perspective, 2x cheap WiFi-capable microcontrollers (e.g. esp8266) will cost the manufacturer something like $4-$5 total for both devices - which is comparable to 3m cable+connectors+cheap chips to handle the cable connection (even ignoring the cost of some person to install the cable which is far more expensive than that) so indeed I don't get why the author considers that doing the connection over WiFi is somehow wasteful.
Is it really a problem? An ESP8266 (which is not the most efficient thing, but is very cheap) uses about 0.5W while transmitting, much less most of the time.
For reference, I have some smart outlets that I measured using a while ago, pulling about 0.05W on average from the wall, or 36Wh a month. That's about as much energy as making 500ml of coffee or charging a phone 2-3x.
not sure if its a problem in reality, radios are the second most power consuming subsystem of my laptop at idle. (first being screen due to the backlight).
ESP32s are basically universal at this point. You can have them for under a dollar if you order in any sort of bulk, and you get Wifi and Bluetooth right out of the box. At this point is more expensive to not use Wifi
I know this sounds pithy (and it is!) but you'd be surprised exactly how cheap and cost effective Wi-fi enabled SoCs are. A lot of the time we're getting Wi-Fi for free, and most of those SoC's don't have the Ethernet controller by default, so it's more cost-effective to use Wi-fi if it can fit your use-case.
Other physical protocols/connection types can be supported of course (I wonder what the longest I2C run ever is), but when you're talking about a retro-fitted client like this is, Wifi or wireless protocols in general are best.