Yeah, HomeKit really does seem much more basic and wary of existing devices. And I’m being very hopeful of how it can add to HA - but that’s not an Apple strategy.
I’ve now read comments in articles saying that HomeKit is not intended for whole home control. Maybe that’s true of the initial offering, but I’ll bet that it is the intention for the long term.
The reason I’ve seen repeatedly for HomeKit not supporting the Nest thermostat is that HomeKit won’t bridge wifi devices because of security concerns. What exactly they’re referring to, and whether that’s the only reason, I don’t know.
There are a few interpretations of bridged devices not being allowed to control the home. You mention the “sensors, not actuators” definition (and I apologize, I’m greatly simplifying it).
One article claimed that controlling light bulbs falls controlling the home, and thus would not be allowed as a bridged accessory. I call BS on that because of the Hue example (Philips one of the early ones on the HomeKit bandwagon, right?). But I could be totally off and that refers to a complete redesigned Hue product for HomeKit.
My interpretation is the bridge/hub distinction where a bridge is dumb, and only translates HomeKit to z-wave/zigbee/insteon but doesn’t add any advanced logic or inter-device logic, instead deferring to HomeKit. But again, that’s me being optimistic. Oh, and devices that provide access to the home must not be bridged (locks/garage door opener).
Apple is all about the user experience. And they’re going to want to control and create that experience from the ground up. Existing devices don’t fit neatly into that strategy, and as much as I’d like otherwise, Apple doesn’t support things that don’t fit their strategy.
But I agree, we truly won’t know what it will and won’t do until V1 is fully released. Even then, we won’t know the roadmap for V2, V3, etc.