Some very well written posts - thanks.
I agree with the overall analysis that the most likely scenario is that the WTBTS will continue to grow , albeit slowly & primarily in 3rd world countries. There will still be this constant tension between getting more hardline in certain areas & easing off in others. As 1914 recedes ever further into the distant past it will be mentioned less & less (though they do have 2034 to keep on dangling as a carrot.)
However as Harold MacMillan said concerning what could happen in the future , "events , my dear boy , events." There is always the possibility of a catastrophic event to hit the WTBTS. We are long overdue some sort of GB scandal. Think of an average BOE of 8 - 10 members. Over a 25 year period one of them is going to be caught having an affair with a Bethel sister (or brother). Someone is going to have some sort of crisis of conscience and leave a la Ray Franz. What about the finances? The governance is very weak - they don't publish any accounts and are accountable to nobody. Sooner or later there may be a whistle blower or government investigation. Or they might just run out of money - already the cash-flow is perceived to be poor.
Perhaps the most likely "event" would be some sort of schism. You only have to read some of the Spanish JW forums to realise the anger felt at the WTBTS selling off the Madrid Bethel , a building that was paid for by the Spanish brothers. There have been problems in Bethels before , such as Ireland & Australia. The WTBTS are closing most of the smaller branches , perhaps partly in an effort to head off this problem and focus resources and attention on the big branches.
Any catastrophic event is going to make many (though by no means not all) to finally wake up & smell the coffee. The WTBTS could be left with a post 1975 situation when hundreds of thousands left , never to be seen again.