This is why a lack of free speech is key to authoritarian regimes. They can keep control even if only a minority of the people are true followers, because the appearance is that everyone is loyal. If everyone else appears loyal because they're afraid of what you might do if they appear disloyal, then you're going to do the same thing.
In such a situation there's no way to be absolutely sure that someone else isn't loyal to the group/leader without exposing yourself to some risk first. Since most people tend to stick with the status quo, and try to minimize their personal risk, that doesn't happen very often. You usually have to get someone with nothing to lose before they'll do that, which in the case of JWs usually means a new convert or someone who's already estranged from family and doesn't have any close friends in the cult. The folks with nothing to lose, therefore never appear to be credible, so they have little impact even when they expose themselves.