Scaling up only works to a certain point. Not every screen is a phone screen and should suffer from the same amount of visible text. If I scale a page to a good line length I end up with text so comically large that only a single paragraph fits on the screen, if at all.

When lines get longer you either have to increase the leading, or font size, or shorten the lines to retain readability. Personally I hate having to detach every other tab from the browser just to change to width to something that's readable. In that sense I much prefer the content to have a maximum width beyond a certain viewport width because it's the only viable option to still have text that is readable.

No, to optimize for larger screens you simply acknowledge that it's the user's machine, not yours, and do nothing. The user will make their own choices. If they want to open multiple windows so they can read multiple columns of text, they will do that. If they want to make the text bigger, they will do that. If they like it the way it is, they'll leave it alone. Not your problem to solve.

A better option is to expand to multiple columns, like Newspapers discovered centuries ago.

That only works when your content fits on a screen because it messes with the scrolling-based nature of the web (or screen content in general – just picture how awkward it is to read an A4-portrait multi-column PDF with fit-to-width on screen).

Personally, I like well-columned stuff switching to scroll horizontally.

(It's one of the things I loved about the design style of Windows 8 apps that has been lost in the Windows 8 hate, and admittedly was a part of the Windows 8 hate, because people don't respect a good horizontal scroll of columnar reading material, sigh.)

I happily browse the Web at 125% on a 27inch monitor because I like pages to be large, but if pages made double width windows have double size content it would be like sitting directly in front of a TV.