I would also like to add a couple of other factors into this evaluation:
1. Little is known about the life cycle of the WTBS. In western countries
it appears to reach a saturation level arount 1:250 of population. This
differs by culture. Also I suspect that the demographics of JW's in
western countries are starting to pull away from national averages - most
conversions occurred a few decades ago, with most new conversions coming
from reproduction. Therefore JW's are aging faster than the rest of the
population, due to not refilling with young converts.
2. There is a global waxing and waning of apocalyptic thinking that affects
their growth. During the cold war, nuclear war scared years, the "end of the
world" scenario had broader appeal. Unknown future world developments will
be another wildcard.
3. As already mentioned, no one knows what the Society will mutate into
over the coming years.
Overall, I believe that thier numbers in western nations will continue to erode
and age, being offset by growth in the third world. Unless they go mainstream,
they will be on a perpetual decline - they are basically near their peak, now. They
will plateau around 7,500,000 members worldwide and then start a slide.