So far with every upgrade they have added features like more powerful processors and power hungry higher density screens. So most people haven't noticed the improvements in battery tech. Now I think phones have reached a point where I don't see many power hungry improvements going forward so we might start seeing longer lasting phones.
That is not exactly true. OLED can be very efficient. Not just in Dark Mode, but you need to be aware of those pitfalls and optimise for it. Since in overall package the Display is roughly the same power but you get much better quality.
Not to mention there are at least 3 - 5 years roadmap of technology in the pipeline that makes it even more energy efficient and thinner. ( Thinner equals potential for larger battery )
The higher density screen might need to require less power but the power to display on them used by the graphics etc is a lot higher. we were on 320x640 screen now we are at 3840 x 2160. Even if the individual pixel takes less power the number of pixel has gone up a huge amount.
