Village Idiot, I was about to say the exact same thing. Many cults and religious groups have come into existence and stayed without a high influx of money.
If the WTBTS were to go completely bankrupt tomorrow, it would not affect their membership one iota, and would in fact probably strengthen it because they would view it as being persecuted in Satan's world.
The Society has a lot of power over their members because the rank and file give them that power willingly.