I think what you're failing to understand is that newer HVAC units are designed to be run in a specific way

Not missing that, as I and yourself said it's done via the actual HVAC unit and not the thermostat

but without proper "eyes" the brains are useless.

No, the nicer features just don't get used, it doesn't make the entire unit not work

there's just too much risk of things to go terribly wrong and multiple people literally die.

Lol, people aren't going to die if you replace your thermostat with an RPI unless you wired the thermostat so poorly that you somehow manage to schock someone touching the thermostat

Again: thermostats just tell the HVAC to run and which part to run, any other advanced features are not required to be catered to in order to work. My brand new fancy schmancy heat pump with all those cool features listed runs off the same $2 Honeywell thermostat my old heater did, if I want the advanced features like zone sensing I can upgrade my thermostat, but my heat pump works without them just fine