I think 60% to 65% of those currently enrolled in the BSA will leave. 60 - 65% of 2.7 million is 1.08 million to 945,000. I would not be surprised if it reached 70% considering that about 30% of the population in America are mentally ill (aka Leftists)
While I would like to see it happen, I don't think it will. The BSA National "suits" did their projections and told us in APR when they announced the new membership resolution, they expected to lose 300,000, or 11.5% of the 2012 membership number of 2.6 million.
Sadly, I think that number may be closer to reality. Society at large seems to have punched the express elevator button to hell.