I really think that the authors here may be onto something important: It is certainly very possible that a fringe extremist belief system like that of the JWs simply has a kind of glass ceiling as some small % of the population who can be attracted to and participate in such nonsense.
Also, like Scientology, the more their bizarre belief and behavior is exposed to the public, the more a significant percentage of the general population becomes aware of their extremism - the more they will have a prejudice against any attempts at conversion. Probably things like this movie director coming out publicly on Scientology (and the Scientology spokesman walking out of a TV interview when he was asked to explain the doctrine of Xenu the alien) - are opening peoples eyes to the Hubbard weirdness. This may be happening in some small way to the witnesses, too.
I know for example that a lot of the people on my Ferrari owner's website in the Politics and Religions section were absolutely horrified to find out about the shunning, the blood transfusion rules (to the point of death of a child), and all the nonsense time of the end prophecy. The owner of the site, a son of a Presbyterian minister, went so far to comment that he always had viewed the JWs as a sort of quiet pentacostal-like church...never imagining what really went on behind closed doors.