Thanks sherherdless - you are an inspiration in terms of collating data and I like how receptive you are to feedback from SBF. More power to you!
My reflections on the trends are as follows:
As with any religious movement, JW organization has a "lifespan" in which it enjoys a heyday (i.e., period of growth that turns out to be its biggest period of growth which in the organization's case in the West is probably the 1950s through 1980s)) then a more plateaued period of growth (i.e., some growth still evident but much reduced compared to the heyday) and then a period of stagnation and decline.
Apart from the Mormons and SDAs, the other nearest example to JW organization I can think of is the millennialist religious movement, The Christadelphians, who are now well into a period of stagnation and decline after enjoying a heyday around the turn of the 20th Century in the West..
This kind of movement lifespan suggests that there is never a "collapse" of membership but a slow trickling downwards. Put bluntly, as a religious movement ages, it loses touch with what made it succeed decades earlier and it gets caught up in preserving itself and combating member apathy - a real problem in an end-times religion that is forever declaring the end is near.
We are already observing that the main source of "new" growth in JW organization are born-ins whereas during the 1950s through 1980s (making allowances for the dip after 1975) most of the growth came from non-born-in converts (e.g., those first contacted through the door-to-door activity).