I fully agree with your assertion concerning the reading of content on the display, and I went over this in my own comment, but I disagree with the other things you suggested.i agree. it's marketing (5.8" > 5.5"). In truth a taller screen after a certain point adds very little utility.
the truth is, its easier for you to scroll down using your finger to read, than it is for your eyes to go all the way down to the bottom of the phone. your eyes stay on a relatively constant vertical zone when reading on a phone. it's not practical nor is it realistic for you to ever use the bottom most part of your screen to read a line on an article.
it's dead space - you will rarely focus your eyes on it, and it is actually very difficult for your fingers to interact with it (instead it is a zone you have to avoid in order to not trigger unintended actions)
the tall screen on the other hand takes away:
1. battery life
2. usability (one hand use, no bezels to rest your finger on)
3. space (no headphone jack!)
not sure the tradeoffs are worth it.
Width matters more than height after a certain point (you fit more words in the same line, or vertical zone). Which is why the 5.5" 8 plus screen I think is a more practical aspect ratio for a larger screen size.
Anyway, you’re right, you’ll simply scroll down as you read, as you do not want to move your eyes greatly. It goes against a basic biological rule that organisms try to conserve energy, and do things in the most efficient way. Human eye musculature does not want to move the eye greatly, constantly, of course. So my assertion is that intelligent design would mean that virtually no app simply shows more content in the vertical plane that otherwise would’ve been scrolled to. Intelligent design would be to keep the content virtually the same, but use the additional space for additional elements/controls that otherwise would’ve required a separate page or just not existed for example.