Newspapers have less than 5% margin for whitespace. they're smart enough to have multiple columns. It's also a side-effect of how every line costs money and they need to cramp as much content as possible in one page.
I get not having read all the way to the end and back, I even get having margins, but it should be relative to the screen size. Fixed width is the issue I think. To avoid paragraphs looking too thin, maybe increasing the font relative to the screen size makes sense? I'd think there is a way to specify a reference screen resolution to the browser so that it can auto increase the font sizes and/or adjust the div's width.
For font size that increases with screen size, you can use some clamp() math, like:
Taken from https://utopia.fyi/type/calculator?c=360,18,1.2,1240,20,1.25...You're not wrong. Increasing font size is one method.
Another method I like to use is to adjust the amount of words per paragraph depending on the medium. I will literally write more or less just to attain my personal favorite of 3-6 visual lines per paragraph.
Or sometimes I will more readily join paragraphs or split them more often in a text just to achieve my target.
Decreasing width is actually just really easy and also works really well when the type of content can vary.
All of this seems like some serious overkill attention to detail I know, but I guess it's a big deal for some people. For example, most people don't really care about dressing nice regularly anymore when they get older and marry because it frankly doesn't matter anymore (and they're totally right), but people who like fashion still care up until the end.