While I'd like to see them slip into oblivion, it's not going to happen in my lifetime outside a black swan event.
They can downsize to almost nothing and still run the religion. Their structure allows for that since the local elders who run the individual congregations are volunteers (not on the payroll) and the buildings themselves are maintained by the local congregations. All they really have is the Bethel's (which they are dumping all over the place), the COs (remember they dumped the DOs), missionaries (they've already stopped sending new ones out so are phasing that out)and branch staff. The printing of literature I think will cease altogether and will likely be outsourced. That way, literature orders could be moved back to a paid structure with the WTS having the local congregation submit orders to a 3rd party and pay for them at the time of placing the order. You could reduce public placements to just pamphlets that can be printed by the local congregations.
There are still downsizing levers to be pulled:
1 - Eliminate COs and turn the position into a local committee of elders. Super easy to do.
2 - Eliminate special pioneers (very easy)
3 - Stop printing literature and outsource with payment by the locals.
4 - Continue consolidating KH & AH. You could do that until you make the religion a web only religion if you really wanted to.
5 - Stop distributing print literature only on-line unless the person wants to order and pay for something.
6 - Reduce staff at branch level to just a few folks. Eventually you could even have branch stuff done by volunteers.
7 - Have RCs in the already paid for AHs or even a web event at the local KH.
Are they in decline? Yes. Certainly in influence and not growing much at all. But, religion is part business and part emotional attachment and when you put emotion and tradition into it, I just don't see it completely going away. I do think it will be a slow decline and could get to the point that they are less than 50k but I don't see them going away (unfortunately).