It depends on what the remaining space is used for. If you have some hidden menu which can be expanded into a sidebar, some vertically stacked content which can

It depends on what the remaining space is used for. If you have some hidden menu which can be expanded into a sidebar, some vertically stacked content which can be moved to the side etc. then restricting the main text to around 80 chars is great. But I would always prefer an extremely long text line over just seeing the background color in 70% of my screen. If the line gets too long to be read comfortably then the user can just resize the window.