I created a report on the 40 people I grew up with. Only about 10 were still JW's by the age of 40, so it exceeded the 66% mark.
I don't see the JWs going anywhere soon. It will take decades. Most of those in at 40 will die in it, because they have too much to lose, so that gives them another 40 years. Even cooky religions like Mormons are still growing in Australia, (out growing JWs), so the indoctrination methods of cults works very effectively.
What I expect is that the internet has slowed growth, and eventually it will halt and start to gradually decline. What really is needed is for the government to introduce into school curriculums a course on critical thinking, teaching how to identify rhetorical fallacies and indoctrination methods. That is needed to help JW youth to recognise Governing Body statements as mind control.