wanderlust: Can someone please explain why the change from an undefined period of time to the "overlapping generation" b.s.?
I had actually typed an answer but lost it; then I noticed that Apognphos said basically what I was going to say.
I'll say it anyway. I think the org realized that without a deadline, that is, with the undefined period, JWs would just go to living as normal humans do. The org realized it needed to keep the excitement up - to keep the carrot dangling - so it had to have a deadline. Therefore, the new generation teaching which has a limit (although it could be decades away).
Now the org can claim the end is nigh, yet still have plenty of wiggle room - plenty of time to gradually come up with plan B when the time limit on this teaching runs out like it did the one that was used for many years before '95.