Very insightful topic and I hope an open rational discussion with lots of input and ideas will help clarify the multiple demographic and other issues relating to declining participation.
As a first comment by me, I see parallels with what is happening in established church membership.  Are there common reasons?  I am not sure.  I do see, however, a better recognition of the "problem" among mainstream religions and a ramping up of their focus and effort to deal with this trend.  Certainly well ahead of what the nudist/naturist community is doing to date.
I will be following this discussion and hope to add other insights (for lack of a better word) as things progress.  Good work Eric, hope to run into you and Carol on the Club O beach someday.  Toes in the sand Oct 13.