I could see that, but you have to remember that the average consumer doesn't have an automation hub or get deep into the automation sphere, and as such many of these products are designed to be standalone devices that can be controlled from a phone. Often enough they can be the only 'smart' device in a home.

That all said, a dumb thermostat like your suggesting would still require a small CPU, ram, embedded OS, and network stack/hardware to operate and communicate, so I don't know how far the savings would go. I can't imagine the processing power on something like a nest is that powerful as it is.