Well , +I don't think either the Watchtower will end, as others that existed before, because as long there is lack of information amongst people, religions spread if we take into account history and current statistics . The more technologically / information based a society is , the less % of persons believe in religions, basically.
I don't think Watchtower will be able to morph into mainstream anymore as any other but instead try to more around in the world to prey on the LESS information based society's. At some point I imagine they will be a small group regarded somewhat odd and stuck in time by people around them. How they will pass from big now to small after I don't know if just falling in numbers evenly with the years or by some blow in the finances like taxing them or something else. Ofc the Corporation will too try to predict they're end as to avoid it.
Either way I think will not end per se , but end as we now it as in relevant surely.
Take Care