I think there is more effort in making other components on the phone more efficient such as the screen, modem, processor, and the OS in general, then there is for the battery.
Not to mention paint-able solar panels are something we will likely see on some specialty phones in the semi-near future to allow your phone to charge while in the sun or direct light. Wireless charging pads may be included as a stock feature. Most likely that will be the next big thing.
Improved battery tech would be an easier fix but if you look up the history of battery tech, there really hasn't been any significant improvements for the last 20 years. Pretty sad, isn't it?