All these repeated readjustment on the word “generation” shows that Jesus never spoke such a sentence (found in Mathew 24:34) because the following verses he purportedly said was about Noah’s flood which has never occurred [Even if Noah’s ark is scientifically possible with a bigger vessel that takes care of 8.7 million species, theologically and logically it is not possible for God to handle a situation that puts greater hardship on the righteous than on the wicked who died in a few minutes after getting drowned in the water]. (Mathew 24:36-39)
This means the sentence “this generation will certainly not pass away until all these things have happened” (Mathew 24:34) is from the thoughtless writer, not from Jesus.
Like any other religion and its scripture, Bible too right only in its essence, not in details. In essence all religions and scriptures are true because they say God exists, and mechanism of ‘reaping what one sows’ too exists. Hence clinging on to details would land anyone into trouble.
Interestingly, Christendom's theologians say gospel of Mathew was written after 70 CE which means after the destruction of temple. Hence writer took advantage of the already-happened-event putting it into a prophetic mode.
Trinitarian verse that was put into Jesus' mouth (Mathew 28:19) shows Gospel of Mathew was written even much later because doctrine of trinity was a later development.