The constant "churn" of membership has historically helped isolate the Witnesses from succumbing to the same fate, I think. It has re-supplied the organization with a fresh set of true believers to prevent it from becoming completely ossified.
I suspect that's coming to an end, though. The growth has slowed down tremendously over the past 20 years. If the constant stream of new believers dries up and leadership doesn't do anything to instill a renewed sense of vigor, I have no doubt they'll eventually go the way of the USSR. Actually, the Worldwide Church of God is probably a better example, but that's a topic for a different thread.