I feel it is important to bear in mind that the WTB&TS membership is not fixed, it is fluid. The Borg as a whole may grow, but each year thousands of people leave for many reasons. Some kill themselves, some go insane and require institutionalization, and some walk away to follow their bliss.
I'm not saying there are not a LOT of "old-timers." but that is more because of psychological inertia than deep conviction. People are loathe to admit when they've made a bad decision. If they realize they are going to die without having a chance to waltz thru the Great Tribulation and Armageddon into "THE NEW WORLD" they might feel they have nothing to lose by sticking around and going through the motions, the way I believe the majority of people of all religions do.
(I feel that most people are CLOSET ATHEISTS, which is exactly what "having a form of godly devotion but proving false to its power" means. I mean, how many people say, "Yes I believe that there is a God, and that he wants me to behave as though he did not exist"?)