> Non-real number example: OG1 was selling 100 books/month by 2012. After piracy this dropped to 60 books/month. For instance, August 2012 had sales of 100 books, and by November 2012 it had dropped to 60 books/month. Over time, this slowly tapered from 60 to 50 to 40 to 30 to 25 books/month by 2015-2016. That's how drastic things were.

If post author had correctly identified the time that the book started getting pirated, the drop in revenue in that 3-month period is much more drastic that before and after, and is the most likely cause (another cause would need to have very compelling evidence).