Does this mean it can't be the reason for WotC decision? As you say, there is an "if" involved, and moreover - WotC might just see it differently, however wrong they are.
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You asked fo a peer-reviewed study that contradicted WotC's position. I provided one.
Anything can be the reason for the decision. Perhaps the assumptions in the study don't fit and WotC acted to the best of their ability. Perhaps there is a new strategy that ties IP into a subscription model and selling pdfs doesn't fit. Perhaps an executive had a blinding hangover and just wanted the conversation done and said "Do it and do it now." Perhaps someone misinterpreted the piracy as theft from the pdf stores and decided to pull all remaining product back. "Why" is the hardest question to answer because unless you are present, you do not even know what inputs were fed into the black box that gave the answer.
All we can do is ask is if the decision seems rational and appropriate in light of established fact and standard models.
The study suggests a revenue impact form pirating <=10%. Is this sufficient to shut down the pdf line? Possibly, for future releases. Assumnptions around quality of provided pdfs and scanned vs. reasons for pirating would affect that decision.
It is established that the pdfs did sell. Any model I can imagine suggests piracy of those products will continue. It is even reasonable to assume that the slashdotting and the wider media interest is likely grow interest in the product.
It is known that WotC will not see any further revenue from those products.
Is removing pdfs from the stores an appropriate choice? I can come up with scenarios where the answer is yes and others where it is no.
The yes answer depends on a shift to subscription vs purchase model for IP delivery. Rationally, you would want the cutover period to be short to limit damage, but other factors may be involved (WotC tends to be over optimistic on their ability to deliver technology).
The no answer requires an over-reaction on the part of WotC either through panic, misunderstanding, or miscommunication between layers.