You can see the same thing happening with the Apple Watch needing an iPhone as what happened with the iPhone needing a computer early on.
It wasn’t until iOS 5 that you didn’t need a computer running iTunes to activate and update an iOS device.
Apple has slowly been detaching the Apple Watch from the iPhone. First with a cellular connection and then with an independent Watch store and adding better APIs for third party apps.
Right now, you can get away with only having the Watch with you in day to day use and the phone company already assigns a dedicated number to the watch (even though you can’t use it), I can see in the next couple of years not needing a phone at all to use the watch.
The Ford Mustang and Chevy Camaro are each better because the other exists. I think Apple users should be especially happy when Google enters the game.
Friend of mine decided to go phoneless -- borrowed an iPhone 6S from his QC department and used it to activate an Apple Watch, which is now his sole mobile device.
I haven’t tried any of the other chat apps. But you can send a voice message with the built in messages apps from the watch. It should work with both iMessage and SMS.