The problem is they are looking at one word, "generation," and we are letting them do it.
In language you cannot understand words in a vacuum. You must understand them in context.
Amen, I say to you, this generation will not pass away until all these things have taken place.
"This generation" from Matthew 24.34 reads in Greek:
HE GENEA HAUTE
Which means, literally, "the generation, this one" or "the generation now" or "the current generation."
This is what it means a few sentences before, at Matthew 23.36:
Amen, I say to you, all these things will come upon this generation.
TEN GENEA TAUNTEN
Again, the expression means "the generation, this one" or "the generation now" or "the current generation."
(The words may seem a little different, but that is because of syntax. In Koine Greek the words are inflectional and change spelling due to where the words appear in context, but the roots remain the same.)
Matthew 23.36 and 24.34 are almost identical. And they both refer to the generation that saw the Second Temple fall. They can't refer to a future generation because Jesus is not referring to the "end of the world" when he uses this expression.
The idea that wars, famines, and earthquakes will mark the end of days is countered by Jesus. In all three records of the Eschatological Discourse (Matthew 24, Mark 13, and Luke 21), Jesus always begins by telling his disciples that such things DO NOT mean the end is upon them.
In Matthew 24, before he tells of wars, famines, and earthquakes, he states: "Do not be deceived." He then ends this discussion about wars, famines, and earthquakes with the words: "See that you are not alarmed, for these things must happen, but it will not yet be the end. "--Matthew 24.4-6.
The same thing occurs in Mark 13, with Jesus stating: "See that no one deceives you...When you hear of wars and reports of wars do not be alarmed; such things must happen, but it will not yet be the end...These are the beginnings of the labor pains."--Mark 13.5-8.
Note especially how it is worded in Luke: "See that you not be deceived, for many will come in my name, saying, ‘I am he,’ and ‘The time has come.’ Do not follow them! When you hear of wars and insurrections, do not be terrified; for such things must happen first, but it will not immediately be the end"--Luke 21.8-9.
In each instance Jesus is recorded as saying that people can be deceived by those who will use events like wars, famine, and earthquakes to declare that "the time has come." Jesus' own words about these: "Do not be deceived" and "do not follow them!"
So the so-called "sign" used by Jehovah's Witnesses to proclaim we are in the end is the counterfeit sign used by those who will mislead others about the end.
In Matthew the only true sign in the first part of the discourse regarding the true end of times has approached is the spreading of the Gospel everywhere. (Matthew 24.14) But it isn't a time indicator. The way it appears in Mark 13.10 and Luke 21.13 seem to resound with Jesus' instruction about ignoring the "times and seasons" and instead busying oneself with spreading the Gospel as recorded in Acts 1.6-8. In other words, Jesus never gives a real means to calculate the end. The comparison of these verses shows that Jesus is telling his followers to pay no attention to such nonsense because they have nothing to worry about but the assignment given.
In Matthew 24.15-28 Jesus seems to be referencing the destruction of the Temple. This section, as you will note, also ends with a warning that his followers not to be deceived that the end of times is upon them, even though it means the Second Temple will be removed from its place.
The Parousia, however, and the genuine end of times appears to be explained in apocalyptic imagery in Matthew 24.29-31. These expressions are very much like one finds in Daniel, Ezekiel, and Revelation, but they are void of the wars, earthquakes, and famines, etc.
The discourse seems to have a primary conclusion with verses 32-35 (and it gets extended further, perhaps by redaction, not merely through the rest of the chapter but filling the following one as well). It appears that Jesus, having just used the expression when in the Temple (as noted at Matthew 23.36) along with the fact that this lead to his eventual discussion about the Temple's destruction, that here Jesus is again referencing merely his own current generation and speaking merely of the Temple's fall at Matthew 24.34.
The truth of the matter is that the Second Temple really did fall during the time of "the generation this one" or the current generation of Jesus. Because Jesus is answering several questions at once doesn't mean all the words apply to both eras, that of the Second Temple's destruction and the End of Times. Nowhere does Jesus ever say that one event symbolizes the other, and there is no text in the New Testament that states the Temple's fall is a symbol of the Last Days. That is a post-Biblical idea.
This isn't to say that the two events might not symbolize one another, but there is no definitive rule in Scripture declaring that they will of have to. The Jehovah's Witnesses have only said they would. But again the Witnesses are also the ones who claim that earthquakes, wars, famines, etc. are signs of the end. Remember, Jesus says they are not. We are being deceived if we believe those who say they are.--Luke 21.8-9.
In light of this one cannot say that such counterfeit signs have anything to do with the words of Matthew 24.34. Events that Jesus say do not mean the end is upon us do not occur over the period of a "generation," neither can a future generation really fit the Greek phrase used by Jesus.