If that text was full width along your screen you'd be wasting vertical screen space instead. Will you ask them to write more to fill your screen up?
What would be nice is a special element <viewport sizeable style=“width:n”> so a user could either enjoy a default width or drag one side of it to resize its margin at both sides, without resizing a window. Or, instead of an element it could be purely a browser viewport-resize feature, like they do for textareas.
Meta: it’s really frustrating that every time an argument about web ui starts, nobody thinks of how to actually make it better and just holds on some particular use case or preference.
Sure there is: Tiling window managers. Available for every desktop OS in some form or another. I assume people who are power users enough to have large, multiple monitors can help themselves.
In my image you can see that either portrait or landscape orientation, there is considerable 'dead' area (edit: either side), by default, even if the text was many paragraphs long.
The usual solution is to smash "ctrl numpad +" four times til things fit perfectly at (usually) 150%. It's actually uncanny how I'll do this without even thinking while browsing.