I think you should think of Apple being the greediest company on earth.
They are already rich.
Yet they charge you a premium and locked you into their ecosystem.

For example, iPhoneX has a larger battery capacity compared to the iPhone 8 Plus, even though X clearly has a smaller body. Why wouldn't they want to increase iPhone 8 Plus battery capacity?!

And yet last year they removed the headphone jack, in which at least they did increase the capacity of the 7Plus battery, but how come this year they have to decrease the 8Plus battery capacity again but still without the headphone jack?!

What I can think of is -planned obsolescence.

Look around forum and you can tell iOS11 is draining all the older iPhone model batteries.