Very few actually pull operating current from the 24v C wire if it even exists on the given system. If it doesn't, R (the switched 24v power for Heat and Cold signals) isn't guaranteed to continually have current. Only when your Heat is turned on (probably a standard toggle lightswitch on the side of your furnace) will there be current on the Rh line, and only when your AC is enabled (possibly a breaker shunt on the side of your house near the condenser unit in a small box) will there be current on the Rc line.
Nest tries to recharge it's battery by trickling the C wire, if available, and if not it will try to charge off of one of the R wires, either during normal operation, or it will try and "pulse" the heat signal to pull a little bit of current to keep going. Thermostats were designed at a time where they didn't even consume any electricity on their own. We're trying to retrofit computers into signaling system, not a circuit.
The GP is right: most new thermostats don't take power from the 24VAC line. That surprised me when my heat wouldn't come on one morning because the battery was too weak to pull in the relay for more than a few seconds. That's what I get for ignoring the "low battery" warning! All my previous electronic thermostats only used the battery as a backup.
In any case, are you really saying that using a toxic metal (mercury), or an imprecise bimetallic strip is really an improvement over a simple $10 electronic thermostat?
A dumb digital thermostat is just a thermocouple and a relay, which you could rig together with very little EE knowledge and a weekend with an Arduino.
It's clearly more work to do it that way, as you'd need multiple "layers" of firmware/code which all need to communicate and run on their own, but i personally see that as insurance against the exact situation you are describing.
Nest is far from what i'd consider a good IOT company. They are the epitome of vendor lock in, proprietary and buggy code, and shitty support.
The thing I don't get with 'control everything with your smartphone!' is that people don't think about everyday use. It's like the people that design these products don't look at the actual, repeated use cases. Why would I want to pull my phone out of my pocket, unlock it, find the app I need, launch the app, wait for it to connect, hit the buttons I want....
(Even when I'm on Android and I can have an IoT control widget on my homescreen, that's still pulling the phone out, switching it on, unlocking it with my fingerprint, finding the page, hitting the button.... oops I forgot to turn Wi-Fi back on, better do that....)
I think IoT is great, but to do a great job at it you need to design the product with that in mind to begin with. The whole architecture of the product has to fit (see again, Hue). Sure, picking an Android tablet is easy, but why would you architect all that complexity? Why not a touchscreen device with a really simple real-time OS that does only what it needs to do?
I'm confident that this will all be self-correcting in the end. Consumers and 'the market' are smarter than we give them credit for. Certainly it takes a long time for them to react, but I think that when enough of the public is jaded by 'bad IoT' and the fad phase has passed, the actually good IoT products will survive and those companies that really think about their designs as a whole will be rewarded.
If it's so easy, why don't more companies do it? Why didn't Nest build their thermostats so that when the battery runs out, it reverts to a "dumb" thermostat instead of turning off your heat? http://www.theguardian.com/technology/2016/jan/15/bug-nest-t...