A good question to ask might be: 'How was it fulfilled in the 1st century?' The Society is applying it to "anointed Christians" in our time who (they say) will eventually "pass away." But 1st century Christians did not "pass away." It was the disobedient Jewish system that "passed away." Christians, in contrast, kept growing after the Jewish system ended. So in the 1st century, "this generation" could only refer to the Jewish generation that rejected Jesus. So how could "this generation" in our time (supposing it were to have a modern fulfillment) refer to those who accept Jesus? This is completely opposite of how it was fulfilled in the 1st century. If it did have a modern fulfillment, you would at least expect it to parallel its 1st century fulfillment, not be completely reversed.
Luke's account, with its surrounding context (Luke 21:29-36) especially highlights the difference between "this generation" and "you" (i.e. the disciples). "This generation" is to eventually "pass away" (vv. 29-32), "But" the disciples are to "watch themselves" so that they might "stand before the Son of Man." Note also the definitive nature of what is going to happen to "this generation": it "will pass away." Versus the tentative nature of what Jesus hoped for his disciples: 'Keep awake so that the day doesn't catch you off guard and you might keep standing before the Son of Man.' Jesus knows, in no uncertain terms, what will happen to "this generation." But he only expresses hope for what might happen to his disciples.
Note also the use of the verb "pass away." Right in the context of "this generation" it is "heaven and earth" that will pass away, "but my words will never pass away." "Pass away" in the context means, 'to come to nothing, be in vain, go out of existence.' The verb (parerchomai; Strong's 3928) is never used in the NT in the modern sense of "to die," as in a natural death. Instead, when used metaphorically, it has the sense of, 'to be destroyed, go out of existence.' (Compare 2 Cor 5:17; Jas 1:10; 2 Pet 3:10; Also compare its synonym (parago; # 3855) at 1 Cor 7:31; 1 John 2:8, 17) The verses at 2 Peter and 1 John 2 are especially interesting (as is also Revelation 21:1), for they show what Peter and John (1st person listeners to the "this generation" discourse) understood to be 'passing away.' They never speak of faithful Christians as 'passing away.'
Prologos, that is a good point that, according to the wording of the passage, some of "this generation" would be on hand to see "all these things." Contextually, "all these things" refers to the events of Matthew 24:15-22, 29-31. But the WT takes "all these things" to refer to all of Matthew 24:4-31. In either case, if "this generation" were as the WT wants us to understand it, then some of "this generation" would necessarily have to be around to see everything happen. This would preclude a total rapture of "this generation" before all was said and done.
Fear of disfellowshipping and being labeled an apostate are the glue that holds this nonsensical teaching together.