I think most JW's didn't become JW's because of the prophecy factor. And most JW's don't care now about the 1975 fiasco. If you see, all the members of the GB and majority members of all the Branch committee members across the world are those who were very well alive and may be pioneering or CO's during the 1975 period. If the ones who lived through 1975 continue to be JW's, then no matter the later generations have no problem accepting it.
For many JW's, they are convinced about their religion more than gravity. A JW may get up from his bed in the morning and ask, "What if gravity is wrong?" but will never ask, "What if the GB is wrong?"