Based on what I know about this types of organizations, I am in fact, amazed. I have to give them credit for managing to survive for so long.
Experts in cults and researchers of these types of groups, all agree that there are two types of cults that tend to have short longevity. One are the type of cults that are based on recovery from addictions. They tend not to last because a lot of the recovery stuff doesn't resonate with second or third generations who have never experienced addictions or its effects, the way people who joined did. Hence, people just don't care much about recovery from an addiction that they don't have.
The second type of groups is where the WT falls into. Those are the groups that predict the end of the world. I guess that unlike other groups that get disbanded after the "end" date comes and goes and they are still here, the WT never gives a specific date. All they do is keep saying that "it's right around the corner". Those groups that predict things and give dates for some major event end, sometimes abruptly, sometimes slowly. The WT has been masterful in keeping their people expecting for an end that never arrives.