I love the photo embedded in this story showing before and after the emergence of smartphones. It tells the whole story right there -- visually.
Wearable is one direction that mobile handset vendors can probably branch out, no doubt. But it may take a few more iterations of wearables to convince handset vendors to follow that path.
That said, I could easily imagine the day when we don't need to carry a mobile handset at all -- because if we want to talk to someone, that modem functionality will be everywhere in cars, in bikes, at home, and on our body...
As is usually the case, the US military has developed some core technology that surpasses what is commercially available, or viable. Here's a pic of my F-35 pilot brethrens' helmet. A major requirement is to give the pilot a virtual realty view looking through the aircarft structure so the pilot can always keep her eyeballs on the target. This means when flying straight and level, you can look through the floor and see the ground!
My vision is that the technology will advance to such an extent someday that you won't need even the wearable devices on your body.
You will see in the air on a virtual screen created by a command from your brain , you will hear without wearing a headphone.
So you won't need a smart phone at all for any communications. Your thoughts will be translated into commands to some wi-fi system and you will receive responses thru the same wi-fi system on your virtual displays and ear-phones.
The iPhone form factor has taken the mobile phone market by storm because it provides an intuitive human interface for viewing and entering information. When virtual displays become cost effective (and we avoid driving each other nuts by projecting our user interfaces on each other's walls) then the display form factor can change dramatically. When voice recognition, mind reading, or gestures are faster than keyboards, mice, and touchscreens then the input side of the form factor can change as well. At that point, mobile phone form factors may become unrecognizable and there may be another explosion in designs. I have a hunch that we are at the pinch point before the next breakthrough. In 5 years we may not even know when people are interacting with their SmartPhone because it is incorporated in their clothing or their glasses.