I think a good analog for the collapse of the Watchtower is the fall of the World Wide Church of God. Founded in 1933, (with a different name) by Herbert W. Armstrong the WWCG shared a lot of theology with the Witnesses. Armstrong's followers believed the end of the world was imminent, and did not believe the Trinity. They also believed in a number of old testament laws (such as keeping the Sabbath) were still required of Christians. Mainstream churches thought WWCG was a cult.
Through the sixties and seventies the church grew rapidly. I can't find the numbers now, but I recall that it was roughly equal to the witnesses, and many of us believed that Armstrong was stealing most of his stuff from us. Following Armstrong's death in 1986 the church hit some hard times. His son took charge for a time but eventually amid a scandal church leaders disfellowshipped him.
Church leaders then set about trying to "mainstream" the church. This led to numerous splinter groups forming. In 1997 the church was recognized by the National Association of Evangelicals. It has about 30,000 members.
I think trying to mainstream the Watchtower would have similar results. The super hard core JW's will form their own groups that still reject the trinity, hell fire etc. Back in the day I knew some elders that would love to install themselves as the new GB. Others will wonder off and do nothing, or find some other church that suits them. The problem being for decades the witnesses defined themselves in terms of what they are not -- specifically nothing like all those other churches.
If the WTBS is to survive it must answer one question: what's the point? It they can't come up with a good reason to be a witness everybody will just go away.