All of the iOS flavors are based on OS X. So basically every Apple OS is a cousin. But each OS has its different uses and APIs since they're modeled on different devices. You're not going to have Digital Crown APIs on anything but Watch OS. Watch OS also isn't going to support quite as much multitasking as an iPhone, especially not an iPad.hmm i mean isnt watchOS currently based on iOS?... and developers need WatchKit to write apps for it.
What if Apple decided to write a new OS from scratch? and we have to learn a different way to write native Watch apps. This could be exciting.