You're going to carry your phone with you anyway.
Even if the phone did have a cell antenna and did have a battery good enough for it, and even if it had human-level natural language processing, you'll want a phone-sized screen for, you know, all the stuff that you use your phone for today, for examples including but certainly not limited to: reading anything longer than an SMS. Looking at a web page. Looking at a map. Playing games. Taking a photo. Showing pictures/video to other people.
The idea that we'll give up on having screens big enough to actually look at things is just crazy. We love our phone screens. The one thing that people demonstrably want their phone screens to be is bigger.
If you're going to be carrying a phone-sized screen with you 95%+ of the time anyway, why on earth would you make the expensive, difficult decision to jam a cell antenna into your watch. Much less all the valid points that people make about voice interfaces and battery and so forth.