Completely disagree with the "killer app" concept. The iPhone shipped without a killer app, and today your killer app that you use your phone the most for is probably not my killer app, or his, or hers.

Same for the watch. Now there it's moved more towards sports and fitness, but originally it was a little bit of everything and gradually it found its way. But again, what I use it for is probably not what you use it for.

General purpose devices have that power. Special purpose "single app" devices do not.