The newer Ecobee’s can run entirely locally through their homekit integration. I tie mine into home assistant and use it that way. I would never have bought the device if that wasn’t available.
If this old version doesn’t have that available then I’m assuming people purchased it knowing that it was reliant on cloud services. It would be nice if they offered customers options besides just letting the device turn into e-waste but you can understand why they don’t want to burn development hours on a device that’s a decade and a half old.
Just another reason to never buy devices that can’t function without a cloud service.