They rely on things being bad enough that additional people will join as a hedge ("maybe they're right, it can't hurt..."). Which is what makes the numbers distressing for them, that during a period when you had a ready-made 'sign of the end' (a worldwide "plague!" a war that might spread! faltering economies!) they could not keep the rank and file engaged and they could not draw in more new converts.
A high control cult doesn't have quite as much control when the members are not within easy reach all of the time. I think that the biggest concern now for the WTS is not the numbers, it is what might be happening among the members who are already in. How many PIMI became PIMO during the past three years? Who will be more of an influence on the rest... the ones who stay dedicated and grind it out every week, or the ones who do just enough to get by and seem less stressed?