In my experience those that were in some way around for 1975 (most that I know were in their teens or early 20s - those of my parents' generation) are the last of the old-school, studious, "scholarly" JWs that actually know their stuff. The ones that stayed after that disappointment are more firm in their faith than those that have come after. They had to learn, early on, to reject contrary evidence and push on with faith, and now it is impossible for them to see that they might have been wrong - they are too proud for that.
The book When Prophecy Fails describes a similar situation with a small cult that predicted the end of the world. When the end of the world failed to materialize many left, but those that stayed in had even more conviction that it was true. In fact, it was only after this failure that they started proselytizing (sound familiar - JWs post 1914...) and the theory goes that it is their own insecurity about their beliefs that drives them to convince others they're right. If you can convince someone else to join your side, then it bolsters your confidence that you've joined the right side.