JW faith cannot become mainstream because its essential foundation and thinking is isolation, extremism, and anti-establishement. Many mainstream religions established and justified their existence by symbiosis with the executive government, as was the case with Roman and Greek Catholicism, Eastern Orthodoxy, or Protestantism. JWs cannot become mainstream, because it will lose its anti-establishment appeal which is praised so much on the WT pedestal. Their faith is not built upon possibility to create executive government unlike other extreme evangelical and fundamentalist religions who would not have problem to establish ISIS version of christian theocracy. Even their version of 1000 millennial reign is naive the best.
JW cult fills the niche in religion demand, where people look into a faith which is not part of any political system. The issue of becoming mainstream religion was already debated decade ago and many ex-JWs believed that it will follow Mormon and 7DA church which are fairly mainstream churches. Nothing of this had happened, and I do not see it happening in any time soon.