What a widescreen monitor doesn't fix is having to move your head to read those 120 characters. With only 80 characters you only have to move your eyes.
For reference: on an iPad, I get 61 characters on a line in the HN input field, and it is about half the screen width in landscape mode. So, an iPad can do about 120 fixed-width characters on a line. I don't think I move my head when reading iPad text in landscape mode.
I expect that people with somewhat normal vision will say the same.