If you've ever had to deal with this in an email client, you can quickly see that 80 is undershooting it in the modern era.
If so, then why not put the limit on line length without trailing whitespace? Because it makes no sense that with indentation I should lose available characters.
> There is a reason why newspapers and well laid-out websites don't have 300 char width lines.
Yes, and the reason is, print and transportation are expensive, so newspapers found a way to cram as much text as possible in as few pages as possible. You don't see them replicating this style on-line, and neither you see it in magazines that are priced well above their production & distribution costs.
The reason "well laid-out websites" don't have 300 char width lines is because web development is one large cargo culting fest of design. 300 may be a bit much due to actual physical length, but your comment has 200+ on my machine and reads just fine.
I don't buy these "80 chars / short lines and ridiculous font sizes are optimal for the eyes" arguments. They fly in the face of my daily experience.
I'm all for decreasing unnecessary cognitive load, there should be quite enough of that without us adding more by accident.