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because the original Christians were told by their supposed omniscient God that the world was about to end
It's not entirely clear to me that this is true. It is certain that early Christians believed in and hoped for the "life of the age to come", but it's not clear whether their belief that it would happen soon was due to something specific Jesus was supposed to have said.
So for example Paul re-emphasizes to the Thessalonians (considered one of the earlier letters) that the nature of the return of Jesus is unpredictable but he also speaks in a way that implies he thinks it could happen in his lifetime. Yet throughout the entire body of work that we have from him (and he's by far the most prolific earliest Christian writer) his practice of setting up Christian communities and the kind of admonitions and encouragement he makes to those communities are at best ambiguous as far as establishing any kind of expectation of the end of the world. His perspective seems to more naturally correspond to Jesus' statement that no one knows the time (but they nevertheless supposed it might be soon) more than an expectation that Jesus had guaranteed it would happen very soon.
Beyond the ambiguous reference to γενεα, I remembered one other passage that appears in Matthew, Mark, and Luke: "Truly I tell you, there are some standing here who shall not taste death till they see the kingdom of God". In all 3 books this immediately is followed by the story of the Transfiguration. It seems reasonable from the way the saying and the event are reported so closely together that they were tied together from the standpoint of the gospel authors (or author if all 3 copy from one source). It's also possible depending on the dating of the text that it could have been written after the apostles were dead, in which case it would be strange for the authors to have written in that way if they understood it to mean the second coming of Christ. That is conjectural of course. As far as I'm aware it's not possible to date that one section of the texts accurately enough.
So, from a textual standpoint it's probably not possible to rule out your hypothesis that Jesus told the disciples the world would end before they died, or within the current "generation", but I think you are stating it as though it were far better attested than it actually is, and other early Christian writings don't seem to support the assertion that they understood him to have said so, since as far as I'm aware they never make reference to such a promise.