I was in the JWs for eleven years. When I left (I was presiding overseer in the late 70's) I did so, carefully as I had some experience of JWs who quit and lost their families (one man I knew came home and the local congregation had, on his wife's behalf, moved furniture into her new - secret - home).
But my family did leave with me, and I too thought that the 1975 debacle would EITHER mark the end of The Organisation OR, just possibly, they would become more main stream.
Neither has happened.
Indeed if you speak to them today (many opportunities here in the UK because they now stand on street corners) they still spout JW prophecy and mock those who thought the end would come in 1975. They claim it was never Watchtower teaching. But my memory suggests that there was an issue of The Awake (Oct 1968ish???) that said that the end MUST come in mid-1970's.