With both the Watch and the iPhone it is about a happy confluence of features which drive adoption. The watch is fitness, it’s a watch, it’s ease-of-use. The iphone is music, it’s the web, it’s a phone, it’s the third-party apps.
So if you think of a set of mixed-reality glasses, there is a lot more work to be done in software in order to fully realise the experience. The software defines the experience of the device here as much as in the phone, except that you’re working in 3D with AR/VR. It’s on the order of designing a new OS paradigm.