There is a very obvious lack of respect for all other possible effects in the reddit post. A good reason for sales to drop 40% would be book sellers not featuring it anymore as "this years new and hot" a year after release. Another one might be the saturation of the market that is the authors larger social circle. Not mentioning such effects at all and just going "sales drop - must be pirates" sounds unreasonable and more like scapegoating. Honestly the whole post seems to me more like a desperate attempt at advertisement instead of any reasonable foundation for discussion.