On one hand, I obviously can’t legitimately expect the manufacturers of these things to support their older hardware offerings ad vitam aeternam.

On the other, it’s a freaking thermostat controller. An appliance. I don’t want to have to replace appliances before they break, and software breaking it is unacceptable.

I’ve been largely holding off on domotics because so much of it reeks of premature obsolescence... Maybe someone over here has good experience with Home Assistant et al?

You could expect them to support the product for the lifespan of the product, though.

I've been using NodeRED + zigbee2mqtt/zwavejs for my locally controlled home automation to great effect. I expose all my devices to Google Home as a convenient frontend, but all the actual devices are mine and locally controlled so I can replace Google at any point. I tried home assistant years ago, but aside from the wealth of plugins I didn't find its approach particularly convincing. I have on occasion used it simply as a bridge between node red and HA plugins before though which is nice for niche devices or integrations

It still works as a thermostat.

I was looking at wireless cameras on eBay and had to stop because I realized I can't just check specs and price. I have to make sure they don't require a cloud connection and or a service plan.

plenty of things should have a mandatory sort of twenty year supportability warranty. not necessarily free repairs but an enforceable promise that a thing / "ecosystem" can be used for a reasonably expected lifetime

for a thermostat that's measured in decades

They could easily license the server component to allow you to run it, or unlock the bootloader so you could run third party firmware.

They've had 10+ years to plan ahead for this eventuality, and it's not rocket science.