The experiences of the 1910s and 1920s taught them not to sound too certain when they made statements. Or at least, they learned that they could make some statements that sounded certain, while also making others that they could refer to later if they were wrong.
This is what happened with 1975. There was a WT or Awake magazine in 1969 that told young people that they would not grow old in the present system of things. Statements like that were forgotten when they had to explain why nothing happened in 1975. Instead, they referred to the statements they had also made, which were much less certain.