1generation2groups made good observation,: Jesus was talking to Earthly Hope believers, like their contemporaries. but According to WT writers, "Jesus had in mind" a greater fulfillment, where the earthly class would never pass away (flesh would be saved), but the anointed would be alive and witness part of the Great Tribulation, but die, resurrect, be gathered, raptured before the climatic Armageddon. So who could be left that will eventually pass away but be kept alive until "all these things" including Armageddon ---occur?
1) not the anointed, they are goners before Armageddon, 2) not the faithful wt other sheep. they will never pass away so. 3) it must be the wicked, anointed or not, that all will "not be gone until" all these things, including the final test occurred. but will be finally gone.
will not pass away--until all occurs. but will pass away.
now--about that snake--