The watch will never replace an iphone.
It’s not meant to. And I don’t believe it ever will. I think with Apple Watch will be primarily a Health device in the future. It started off as a novelty item and primarily communication device handling certain features like Apple Pay, controlling camera functionality and even remotely opening your garage door. I think Apple Watch will always be dependent on the iPhone aside from LTE for certain things, but remember, it’s an _accessory_ to the iPhone.