As a kid we had a very simple (mercury) thermostat which had two levers -- one for heating set-point & one for cooling set-point. I recall it working quite well.
In effect, two strips of metal were bonded together forming a coiled "spring." Because each metal had different coefficient of thermal expansion, the spring would expand or contract to different lengths depending on the temperature. Place the electrical contact at the temp where the heater or HVAC should turn on & bam.
No electronics at all, just a temperature-dependent mechanical switch.
I had a problem with the temperature in my house often being wrong, so I bought a smart thermostat. Now I have ten problems.