Piracy of comic books can have both positive and negative effects depending on if the work is ongoing or complete, according to an academy study from Keio University Economics professor Tatsuo Tanaka.
Tanaka's research was spurred by a masive piracy takedown campaign in 2015 by a consortium of Japanese publishers named the Content Overseas Distribution Association (CODA). The professor studied 3,360 individual manga volumes and their sales before and after the takedown.
“Piracy decreases sales of ongoing comics, but it increases sales of completed comics,” Tanaka states in his paper. “To put this another way, displacement effect is dominant for ongoing comics, and advertisement effect is dominant for completed comics.”
The professor postulates that since most publishers promote ongoing titles more than completed ones, leading readers to "almost forget" them, piracy acts as a de facto means of a reminder that stimulates sales.
You can read Tanaka's full paper here.