One thing to consider about the emphasis on 1975: it was said to be the year when 6,000 years of man's existence would be reached. What significance did the WTS ever give to that claim, that made it matter to the rank and file? Even now, what does it matter that 1975 marked 6,000 years since Adam's creation? Do they ever make any reference to it as meaning anything?
The only significance that was ever implied was that it could mark the year when Jehovah would finally bring about the end of the current system of things and usher in the new world under divine governance. Otherwise, what did it matter that Adam was created 6,000 years prior to 1975? How would that affect anyone's decision to join the JWs? Why would they care? Outside of using it to push the concept that Armageddon was almost here, why would a person care about Adam's 6,000th birthday?