Phones are already more efficient but the improvements are eaten up with larger screens and higher performance processes that do more and are used more. I.e. part of the reason a flip phone battery lasted a week was that you didn't use it with its display on 5 hours a day.
The day will come when screens are very efficient and batteries will be better too. But will the rate of improvement be rapid enough to keep up with screen size and performance / usage increase?
Overall I think the smartphone has been commoditized. Not a lot of excitement about new models. The only good thing for the industry is that smart phone use is very widespread, so it still is a decent market, if half the worlds population owns / uses one. I checked, and it looks like almost a third of the world already has a smart phone. That's amazing.
https://www.statista.com/statistics/330695/number-of-smartphone-users-worldwide/
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