I think Simon's point is well made. The generation teaching is a big thing for me but for many they simply do not care. The GB play the game very well and change doctrine in a way that people can digest with very limited collateral damage.
The organisation is held together with the glue of cognitive dissonance and it takes something very strong to start to break that down. In my experience, for long standing Witnesses it's injustice and personal matters that generally allows any dissatisfaction with doctrine to turn into progressive and open minded questioning.
I would not underestimate the leeway the GB has to be able to change huge amounts of doctrine with very little real damage to the faith of the vast majority.