Lots of people are using split screen editors though. Tbh if you are butting up against the 120 limit, you're probably doing something wrong and could easily break the lines, or use less levels of indentation.

Over 120 and most likely something is wrong with the code. That many levels of indentation usually means that code's cyclomatic complexity is off the charts and is in desperate need of refactoring. It could indicate bad naming practices (to long) which also should trigger refactoring. This is all to say it is more about what long lines say about the code than anything to do with the display.

Some languages are just naturally worse at this (eg. Java), so there is always some flexibility. But for most languages that don't have multiple levels of indentation by default and the custom of overly long names, 120 is more than enough to be a good guidepost.

I have three 4K monitors and use a small font and think 180 is way too large.

Long lines are less readable and I want to have several terminal panes or editor windows side by side.