How will we measure Apple’s Watch success? by Horace Dediu at asymco:
Dediu has a total of six thoughtful vectors by which success may be judged, but these two stood out.
A measurable and significant reduction in the use of the iPhone. The Watch peels off uses from the iPhone and therefore the more it peels off, the less remains. However, that which remains will be more uniquely valuable to the incumbent. This is the process of carving and erosion that the PC experienced vs. mobile devices in general.
Setting the reality that no one outside Apple will likely be able to measure iPhone usage at scale, the anecdotal perspectives of Apple Watch users are likely to paint an interesting picture. Beyond the product marketing and press headlines about saving time and displacing the phone, the experience of ending the day with noticeable surplus of phone battery life will be the quickest way to note value of the Watch. After that it may come to comments from those people around the user, who begin to notice behavioral changes. Fewer moments of the phone interrupting the user’s activities ought to connote the value of the Watch.
An increase in the mix of large-screen iPhones. As iPhones are removed from pockets more rarely, the larger version might be more comfortable to carry and more useful to use for the immersive tasks that are outside the scope of Watch.
The idea that Apple Watch will drive users toward larger phones is logical; the differential in size will make the phone feel more appropriate for longer engagements where greater visual real estate is useful. Also, the Watch may drive down usage of the phone while the user is in motion, which may in turn encourage users to carry their phones in more secure ways, thereby accommodating larger phones. However if the Watch is adopted aggressively by the developer ecosystem, and Apple addresses the natural shortcomings and critiques of a first generation device, there are good odds that small phones may see a resurgence amongst those users for whom the Watch becomes the primary device.