The Apple Watch does not fill a niche
The Apple Watch rather than fill a niche provides a fifth screen. According to Wikipedia the four first screens are the cinema screen, the television screen and the mobile phone and tablet screen. The fifth screen is the smart watch as designed by Apple, Samsung, Sony and others. Apple and others have designed phones that bring the mobile phone experience to the wrist. Energy efficient Suunto, Garmin, Fitbit and other brands fill the wrist worn niche effectively because they have designed devices with energy efficient displays that provide tracking whilst at the same time giving extended battery life.