I can resize my window easily if I wanted shorter text. Or used ctrl-shift-m on Firefox. But I can't easily make the text longer without userscripts or addons.

> actual user studies to show that wider text is harder to read

That may apply to most people, but not to everyone.

afaict it applies to literally everyone. there's a variable "sweet spot" of course, but once you get out to "extremely wide" it's reliably worse for everyone, and there are LOADS of computer monitors that qualify for that label.

margins to control the width of large blocks of text have a ton of research in their favor, it's not just "more whitespace = more gooder" UI design madness. there's some of that of course, but there's a sane core underneath it all.