Job isn’t always to score but to make life tough on the opposition for a shift while the top lines take a rest. If you can get scoring through three lines, you’re doing well.
Teams will still avoid putting too many top guys on one line (eg McDavid and Draisaitl) because it concentrates scoring too much on a single line. But then if they really need a push, they’ll concentrate the firepower onto one or two lines and start double shifting.