Hi sbf, I haven't read anything by David Voas. I looked up some of his articles, and they look really interesting. Unfortunately it seems you have to pay to read the full articles.
I agree that it will be interesting to see how they handle decline. Some here argue it will be interpreted as a sign that the end is near. That might work for a short while. However, it seems to me that JW's generally struggle to justify why their religion is the one true one, these days, and one they seem to turn to is bandwagon fallacy (ie that 8 million others believe, then we must be right). It will be fascinating to see how they react if that bandwagon starts rolling in the wrong direction.