Here's how I think it will end: One day there will be a huge Watchtower scandal that will make most if not all the major news networks' top stories. This will result in many JWs hearing it and becoming disillusioned. Also, "people in the territory" will hear it too and will confront JWs with it everywhere they go knocking on doors. A large segment of JWs - maybe half or more - will leave.
This news would be so scandalous that it will forever tarnish the image of JWs so that no one "in the territory" would consider JWs a viable religion, much less the only true religion. And so over a period of years or decades the remaining JW population would stagnate at first and eventually it will start dwindling slowly as they die off, until there are just a few thousand left scattered across the globe. By then Watchtower would be out of business and the last few die hards would be forever going over older literature last published decades ago, until eventually there are no JWs left.
In the midst of this crisis, some members of the GB might split off and create a new sect under a different name. The new sect might then either be JW 2.0 or an entirely new, fully reformed religion that discards all the cultic rubbish of the JWs.