Optimizing for super-rare users who have a deliberate desire for long lines is less important than optimizing for the vastly-more-common case that the browser's size is large as an artifact of content the user was viewing on a previous site, but the user still prefer a sane line length for text content.
That's odd. I wonder if these people that like looong lines are mostly younger people that have read more from screens than books (but not so young that they've read more from phones than computer screens :))? A book with very long lines would be super weird.
Agreed. I'm specifically referencing wasted space. As I've gotten older, that "wasted" space gets replaced with zoomed fonts using ctrl+=. I've found this attitude and use isn't all that unique anecdotally, so I found the assertion that it's "rare" to be contrary to my own experience. I have no citation the other way though!
