It would have to get to the stage I think where there was a 'critical mass' of leavers.
If 1 person in a KH leaves then so what? but when 5 ro 10 leave they have a problem.
When the ones left start to feel like they are the minority then I can't see them sticking it out much. Would YOU have stayed in your KH if half of the people left?
The JW's today are not the zealous, steadfast group they once were as they now live on a watered down set of beliefs and many are really only in it for the social aspects.