It'll be a whimper. Although the end of the WTBTS has already started, there's still a lot of mileage they can get from reinvention, i.e. new light. But the old guard isn't getting any younger. When people my age finally ascend to the absolute power of the Governing Bawdy, they'll have had several years of seeing the original tenets of the 1914 generation get vaporized and become meaningless. And at some point someone's going to say it's time to reinvent the whole template. But they'll do it as they've always done it: gradually, in subtle steps.
But I don't see JW's 100 years from now in the same way they exist today.
The only way it could end with a bang is something catastrophic that will upend their whole belief structure and break through the cognitive dissonance of their followers. It would have to result in the total demoralization of their members. Like Jonestown. I don't see that happening. I do, however, see the probable end of the whole thing, and it's a long way off.