Or make the device as SIMPLE as possible.
The thermostat should be a remote display and a sensor. The real brains should happen on a controller near the heating system, that can be swapped in and out as time goes on.
As is the thermostat is a set of brains that can be swapped in and out as time goes on. I just swapped two thermostats in two homes a few weeks back and it took <20 minutes each with no prior knowledge, even though in both cases I had to swap the backplate.
What is the benefit you see of moving the swappable brains closer to the heating system? Doesn't that actually make the whole system more complex because now you have two separate devices where there is currently one?
The controller for the HVAC system costs a heck of a lot more than the thermostat.
The thermostat is the controller is most simple HVAC systems. The evaporator and air handler will have some "brains" re-compressor cycling and potentially fan speed, but the thermostat is calling for heat/cool.
You mean condenser instead of evaporator, the evaporator is inside the furnace and has no controls as it’s just a coil. The outdoor a/c unit contains the condenser and compressor and has the contactor (and controller if it’s an EC motor) for the compressor motor. The contactor control coil is 24v and the contact is closed when a call for cold is received from the thermostat.
Newer A/C units have EC motors with variable speed motor controllers and newer furnaces will have an EC motor blower fan and lots of factory control points wired up to a controller, mostly for the burner (and related gas/burner safeties).
Still, even in newer furnaces and a/c units, the thermostat is the main controller as like you said, the calls for heat/cool are the only input to the system.
Commercial HVAC controls for air handlers, boilers, and chillers have a lot more control points, an AHU can have 40 or more control inputs and outputs tied to it. Discharge and return air temp, multiple differential pressure sensors, static pressure, outdoor air temp, outdoor air pressure, duct avg temp, etc. These systems are much more complex than any home HVAC setup and usually have multiple controllers networked back to a master controller which orchestrates everything.