The overlapping generations will be the eventual undoing however it take 50 years or more still. The current GB won't have to worry about answering for it which is sad but eventually with so much hinging on 1914 even if the overlapped contemporaries don't reach max age I think as that clock winds down you'll see many completely lose faith.
I think we have a tendency to be convinced by things that convince us ... and mistakenly assume they will be as convincing to everyone else.
Why did we leave? Really.
Was it really some doctrine? Or was it because we were treated badly, didn't think that smelt like "gods organization" and that thought allowed us to start questioning things. Did we discover the doctrinal gaps and flip-flops along the way to leaving, as a result of researching to put our mind at rest or looking for proof to convince others to leave?
Until and unless people have some trigger to doubt themselves and question their place in the organization, they don't really care about the doctrine.
It's religion - none of it makes any sense. The JW beliefs don't make any less sense or are any more incredible than the idea that a super being inseminated a virgin jewish woman so a hybrid creature could grow up to be executed and a book written about it (after some super-hero exploits along the way).
It's all complete and utter nuts. Adding a few extra other things that don't make sense doesn't suddenly make it topple over - it's not Jenga, all perfectly balanced until the generation teaching or any other is added. It's a pile of sticks on the floor and different faiths just have a slightly different pile in a different order.
The truth is, there is no spoon.
(yeah, I didn't have a proper ending and wanted something profound)