Watchtower is not like the small Doomsday Cults like Jonestown, Heaven's Gate and the Branch Davidians. They're too corporate with lawyers and such to become a suicide cult. In order for Watchtower to become like that, they would have to shed the corporate structure and lawyers.
Watchtower would need a strong figure-head like Rutherford or Franz would have been had he not been so old or held in check by the newly formed Governing Body.
As far as changing doctrine to reflect a suicide cult's thinking, that wouldn't be so hard to do. JWs have been taught from early on to expect "new light" at any minute. It wouldn't be that hard to introduce this "new light" as a new way of being a true Christian.
What would be more likely is a break-away group or a renegade Kingdom Hall to form a suicide cult. I could see that happening. And could likely happen should the Watchtower Corp. rot from the inside out.
But as far as Watchtower Corp.'s demise, that will be a slow demise at best.