It really seems dumb to have a thermostat at all. If I have a smart home, why not just have an app for controlling temperature settings? With a google home, I should be able to say "Set temp to 21".
As for having a thermostat, there needs to be something that actually has a thermometer in the house.
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.
Not to mention the security implications of having an appliance on your network.
Yeah, but the Nest is already connected.
The reason you need a fallback local control is because, in many environments, heating/cooling control failing is a dangerous condition.
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.
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.
Btw, a nest is about $200, a furnace is $2-5k.