> The sandbox marks the end of the open system.
The existence of the sandbox doesn't. Not yet. As long as there's sideloading, it's still an open system.
I'm very happy with my ability to sideload apps on Android, and it keeps the platform from being too locked down. I can install a whole alternate app store from Amazon that way if I want -- in fact, it would be great if Cydia or someone released an open app store for OS X apps that don't meet Apple's criteria. You wouldn't have to jailbreak your computer to use that app store or any of its apps.
That's the barometer for when Apple goes to far. It's possible that day will come -- when they've gotten this far, it would no longer be shocking to see them go the rest of the way. But until they take that final step, OS X is still an open system.