It's not like unused space results in some kind of waste, this is not paper, blank space is okay
That's a pretty subjective view, and one at odds with literally every other website I can think of
To be fair, I read fairly quickly, so maybe this is true if you only read 100 words per minute or whatever.
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.
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.
What happened to 80 ? Ok, 75 is "close enough", but 55 ?