We can look at history for an answer.
After 1925 failed to produce the promised resurrection of David, Solomon and a host of other "ancient worthies," the membership of Jehovah's Witnesses declined by about a third, if I recall correctly.
Then it experienced decades of growth.
Why? Because they offered a serene vision of a utopian future that is just around the corner combined with a hellish vision of punishment for all God's (read: YOUR) enemies.
After 1975 came and went so unceremoniously, the membership of the JWs experienced a large decline.
It has since experienced a spurt of growth to new heights (that seems to be slowing again).
If the WTB&TS announced that they just got a phone call from God that the big A was scheduled for 2150, a large number of people would probably leave the organization. Others, feeling that they had already invested most of their lives, would continue to abe "faithful," thinking they were passing some kind of cosmic test, and new people would join, for reasons I cannot imagine.
Humans are not rational, and tend to expend large amounts of energy justifying their irrational decisions. You will never go broke underestimating the human race.
P.T. Barnum was right. There's a JW born every minute. All the WTB&TS has to do is find 'em and fleece 'em.