Since you quote Luke 21, note verses 22 and 32. Jesus insisted that ALL prophecy would be fulfilled in his own generation. Doesn’t ALL mean ALL?
I'm not sure I understand the point you are trying to make. All the events related to the destruction of Jerusalem in A.D. 70 did take place within a 40-year generation of Jesus saying these things. If you are saying that Jesus (or the apostles who supposedly put these words in Jesus' mouth) was wrong, mistaken, for predicting that his return and the end of world would take place within 40 years, then you have a different view of Jesus and Scripture than I have. And Jesus is not predicting here that his return and the end of the world will take place within 40 years.