I have no doubt F.W. Franz and WT leaders in general sincerely believed the 1975 thing. The idea came up in the late 60s, during a very unusual period in the Western world, combining a lot of anxiety and enthusiasm, when 5-10 years away was "long-range": anything could happen.
I think the coincidence of two independent (although both goofy) WT calculations was just too big a temptation for a "no-coincidence" mindset to resist. People who were 20 in 1914 would be 80 in 1974 (all by the old WT definition of "this generation"); and mankind would be just 6,000 years old in 1975 (by WT calculations again). The two theories came too close not to attract and confirm each other in most WT minds.
We became JWs in 1972, in a congregation which was quite assertive about 1975 (I remember asking an elder, "what will you do if it doesn't happen en 1975?" and he answered "I don't even want to wonder about that: it will happen in 1975"); my father would always point to "nobody knows the day and hour" "at the time you do not think" -- and he appeared as lacking faith, even in my own eyes. Eventually a new congregation was created where we lived, and it was much more moderate. The last end-time frenzy was in 1973-4 with the first oil crisis (my father bought a lot of food stock which was lost). But as the date approached it was, in effect, forgotten. Too close, it became too clearly irrealistic. I remember I realised the date was past when I heard an elder friend joking on "Exactly 6,000 years ago, Adam had been in his garden for a couple of weeks, and for the first time he saw a lion's tail."
I know the statistics indicate a big fall in numbers worldwide after that, but I can't remember anyone I knew leaving for this reason (not even in our first congregation). I for one enjoyed the new direction of WT literature from 1976 on -- less end-time scenarii and doctrine in general, more "spiritual". That was during this period that I, too, left high school to "pioneer"...
I suppose this has affected my approach to "Bible prophecies" as well, because I know from firsthand experience how "faith" which starts with eschatology (end-time expectations) can survive failed prophecies and more generally outgrow its original setting completely.