To be fair, Denny O'Neill and Neal Adams brought Batman back to his darker roots, after all the Silver Age silliness, before Frank Miller did.
There's a tendency to simplify the narrative by asserting that Miller single-handedly rescued Batman from Adam West campiness with The Dark Knight Returns, whereas that process had already begun in the 1970s, under O'Neill's watch. Turning the Joker back into a homicidal maniac, etc.
(It's perhaps more accurate to assert that Miller changed the general public's perception of Batman rather than the actual content of the comics. Batman hadn't been "Zap! Bam! Pow!" for some time at that point, but it took The Dark Knight Returns to get mainstream media to notice that.)