I don't think the WT will die out completly, there will always be a hardcore. Congregations will become static in their members, then gradually start to shrink as younger ones turn away and leave.
I remember watching a program about the Exclusive Brethern. One of the things that puzzled me is that at meetings the men are not allowed to wear jackets or ties, it was never explain why. One of the other things was they owned a house were if a person had broken one of their rules they would be put in isolation, not allowed to leave the house and only visited by the Elders. One woman who had left but wanted to return, spent 6 months alone in the house.
I know someone who left the EB when in his early 20's. His parents cut off all contact completly for 10 years. In that time he got married had children, but still no acknowlegdment from his parents. Then one day they contacted him saying the rules had been changed. They could visit him, but he couldn't vist them. Also they were not allowed to eat or drink while in his house.