I have a "wireless" thermostat (Honeywell Redlink) and there are a lot of problems you have to think through to make it work. In particular, what is the failure mode when the thermostat can't communicate with the furnace? What if your internet goes down and it can't communicate with your phone? You might need a different failsafe for when it's 0C vs 25C outside, but what if your exterior temp sensor stops working or can't communicate with the furnace? Not to mention the security implications of having an appliance on your network.

All these problems are made much simpler with a wired thermostat and battery backup. I wish it was easier, but in this case "smart" doesn't really buy you much energy or UX savings over a simple programmable wired device.

If the Internet goes down, you could still connect locally. It could even create its own wifi hotspot, like many devices nowadays.

Not to mention the security implications of having an appliance on your network.

Yeah, but the Nest is already connected.

The thing hanging on the wall is a receiver for remote control and a fallback local control.

The reason you need a fallback local control is because, in many environments, heating/cooling control failing is a dangerous condition.

At the end of the day, something has to be wired to the A/C to provide control and the majority of houses today are already equipped with that wiring inside the home (usually into a wall). I don't think there's any rule that says you have to have this in a wall, but I'd be willing to bet most people will prefer having that fancy smart thermostat mounted in the same place the old one was (myself included).

If nothing else, I think it creates a sense of security in that they'll be able to still use it if they lose their controller or their phone dies.

But I like jbob's idea.

You can cut a huge chunk of the cost out of the Nest if you drop the LCD, touchscreen glass, and fancy stainless steel housing.

It could be a simple plastic box with the CPU, radio, sensor and control relays inside, and then a hidden mechanical thermostat knob/control to use as a floor + emergency override if the internet cuts out or the phone goes away.

That's what I was getting at! (in an admittedly crass way). Split the device between temperature sensing and temperature control.

That said, there are devices that come extremely close to this, like the RadioThermostat CT-50. It has it's own HTTP API so you technically could operate it on your own.

Because home control / voice control is already established (Google Homoe, Alexa, etc). Why would I want a single voice controller when I can leverage the mesh of devices I already have around my home?

Then how will Nest sell you a high margin locked up gadget?