I'm going to want to carry a "phone" for sure. Most people who've ever had a smartphone expect to have one the rest of their life.
I'm also going to want (at least) a machine with a keyboard and bigger display than my phone.
Why in the future would I want to tie one to the other? I wouldn't want that today — my computer works just fine without having to do some weird sync/plug-in dance with my phone when I want to use it. Even if this was seamless and even if we ignore concerns like finite batteries, what would I have gained besides a single point of failure?
I of course would like pervasive data sync. But why would I want the hardware of my computer to rely on the hardware of my phone?