[stinos]

I mean, why not just have HA just control a hardware thermostat that has different mode settings (home, away, etc.)? Plenty of them exist and its, honestly, easier than trying to make HA into a thermostat.

Wel yes, that's exactly my point: when done like that there's a watchdog in place. My issue (or rather reluctantce to use it) is that HA and the likes provide as far as I'm aware no watchdog features for systems which don't yet have it. Think some hardware AND-style gate which is only going to apply HA's last state if, say, HA can prove it is up and running properly.

[marcinzm]

There isn't one because the by far most common failure case isn't "HA isn't running" but "the communication protocol isn't working." A watchdog must be on the device itself because otherwise it's basically useless for all intents and purposes. That said it also doesn't in my experience matter in like 99% of situations (a light stays on... who cares and you'll directly notice the issue with your eyes anyways) and for most of the rest you can just have HA send you a notification that it lost contact with some device for too long. You want notifications anyways because you cannot assume that a thermostat being on means the heater is on. If you need a heater to avoid a water pipe bursting then you should have a low temperature alert as well. The breaker may have blown, the relay may have broken or the heater's overheat fuse may have blown at some point.