The next evolution in smartphones is probably a battery that can last as long as your old simple cellphone, but still allow you to do all the powerful things your smartphone can do.
I agree on this point.
All of the smartphones that are at 4" + for screen size are really about as big as they can comfortably be. Mine is 4.3" and is at the limit for what will comfortably fit into a pants pocket and thickness is not really the determining factor here.
An extra mm or two of thickness for significantly extended battery life would be a big deal for a lot of people.