I think it depends on the individual.
Why did they join the religion: for intellectual reasons or for the camaraderie? Were they born in? Do they actually believe it's the truth or are they less concerned about doctrinal details than they are about the supposed evidence of this being God's organization? How happy are they with the religion? Do they harbor doubts? Have they had run-ins with "prominent" JWs? How comfortable are they with allowing themselves to be deceived?
I think a forceful intervention might be successful on someone who has been conned into believing that the JWs have the truth. Forcefully making them see that all is not as it seems could have a powerful effect on breaking the religion's control over them. Forcefully teaching someone about the fallacy of the 1914 teaching, the false 607 B.C. date, the UN scandal, the child abuse cover up, the way the doctrine has completely morphed over the years, among other things, could lead a devout JW to reassess his faith. Unfortunately, I think that few modern-day JWs fit this category.
Many of today's Witnesses, at least from my experience, seem to be people who don't care too much about doctrine. They just accept that it's the truth, either because they were raised that way or have invested too many years of their lives for them to consider the alternative. It would be much harder to deprogram these people because they are not under any illusions about the JW leadership. They remain in the religion for social reasons more than anything else. They've developed a tangled web of social and family relationships. Leaving the religion would be more of a hassle than remaining in it thanks to the religion's excommunication policy. There really isn't much incentive for these people to leave. Many of them lead double lives anyway, so there really is no reason for them to leave and be shunned by everyone they know.
The goal of an intervention, as I understand it, is to give a cult member information about the cult that they otherwise would not voluntarily consider. If a cult member has been genuinely deceived into following his religious leader and the cult member later finds out that they were lied to, the logical reaction would be to leave the cult. But how could this work if the person in the cult isn't in it because they believe in the cult's teachings but rather because they enjoy the social atmosphere of the cult or are unwilling to face the consequences of excommunication?
It's an interesting and complicated question. I think that a forceful intervention might have been successful on me because I genuinely believed in everything the Society taught. For a few years, I didn't have a single doubt. I didn't have access to any critical information. Nor would I have ever considered reading so-called apostate material. Had someone forcefully shown me what I now know, perhaps I would have left the religion many years ago. Of course, there's no way of knowing how I would have reacted.
There are many JWs that come to mind on whom an intervention would probably never work. Some of them are people who have so completely bought into the religion that they would support the Watchtower despite what they might learn. They've convinced themselves that if they have negative thoughts about the Society it's because Satan is putting that thought in their head. Nothing they could possibly learn would deter them from their path. Others have invested too many years into the religion, and, mainly due to pride, could never possibly bring themselves to recognize that their lives were spent in working in vain to bring about a new system that will never come. Others have grown up in the religion and are deeply imbedded in the JW sociological structure. They get their sense of satisfaction and personal fulfillment from their status in the religion. They enjoy the attention they receive because of it. Many of these people are completely comfortable leading double lives. Whether the religion is or isn't the truth isn't much of a concern to them. What matters is that this was the life they were born into and they are going to play by the rules and exploit it as much as they can. Putting up with the consequences of being disfellowshipped is just not worth it to them, especially since a lot of these people married young and now have spouses and children in the religion.
