The only years that I'm aware of in recent history that the JW's lost numbers was after the 1975 debacle. Someone posted on another thread about the damage control they're doing with the new "generation" nonsense with an assembly part where one person explains the "new light" to another that isn't too sure of it.
If they're having to do damage control like this over something relatively minor in comparison to 1975 (just some more "new light" as always), how do you think they would hold up under another 1975-level "oopsy"? People seem to get less and less willing to be locked into religious systems that just don't conform with the modern world as it is.
If the internet was around, how many more people do you think would have google'd "jehovahs witnesses" at some point in 1976, perhaps while alone on the computer after the world didn't end? News and information wasn't as widespread back then either. Look at all the eyeballs that recent loon got when he was predicting the end of the world based on the bible. He was all over the news.
Granted, the Washtowel Wizards won't make that mistake again, but they are faltering already on a smaller scale. Who's not to say that they'll actually be nothing more than a footnote in history if the already slowing growth doesn't improve and the mid 2000's roll around with the second of the overlapping generations starting to die off en masse? I don't think that they could survive another hit like that in the Internet age.