> A4 proportions are objectively not great for reading (too elongated).
This statement is purely subjective. It is false in general and there's nothing objective about it. I've read thousands of pages of A4 with no problems, and in fact I'd say it's great.
No one mentioned books, but standard paperback book sizes are actually even more elongated than A4 (but smaller). A4 is used a lot in academia for papers, theses, lecture notes, etc., and that's mainly what I've read in that size.
It's not about the aspect ratio, but the total line length measured in characters. If you pay attention you will realize that most published papers either have two columns or large margins, like the standard LaTeX article layout.
