The only generation that could not pass away are the wt "other sheep" They are the millions that wt predicted in 1918 that will never die, never, not pass away.
To nail a witness on that, ask , to start, the defining question: did "all these things " include the destruction of the temple, of Jerusalem? Did all these things include all "tribulation(s) since the world's beginning" read Math 24:21: Did the total destruction during the flood happen since " the world's beginning"?, so, do you agree all these things must include Armageddon? will the other sheep survive Armageddon?-- er-- yes, will the anointed generation survive Armageddon--er-- . no, they will pass away to heaven. ask your elders about that.