And here I am, comfortable with and prefering lines at or above 200 characters. Given how many other changes have been made in the name of UX that have been gone compleatly against what those letters stand for, I'll press X to doubt on that one.
To be fair, I read fairly quickly, so maybe this is true if you only read 100 words per minute or whatever.
Short columns aren't (only) about overall reading speed, it's about comfort.
When you're done reading a line your eyes must move back to the start of the next line. With short lines you can still fully see the end of the previous line so it's easy to locate. It's a lot harder to do with long lines.
I don't understand this logic. If I was uncomfortable using the entire width of my monitor, I would have gotten a smaller monitor instead. I find moving my eyes less annoying than moving my fingers to scroll; I often don't even rest my hand on my mouse or keyboard when reading, because I can use my hands for other things at the same time, or even just rest them somewhere else.
My point isn't that everyone will agree with me, but that all of these experiences are subjective. The only actually objective signal that the UI developer has is the width of the screen, so that seems like what should be used. Some amount of margin isn't unreasonable, but asserting that my display is "too large to be comfortable" is dogma, not science.
This doesn't reflect my own lived experience.