Leaving:
There's a couple ways the argument could be made. JW's base there salvific model on Revelation 7, which mentions only two groups that attain salvation.
Therefore during the "Time of the end" you are either one or the other. You are either one of the "Great Crowd" or you are one of the 144,000. There is no other option or third group.
The idea of "Other sheep" living during the "Time of the end" who are not also prospective members of the "Great Crowd" is currently theologically impossible. This is difficult to argue even with an active JW, but I can provide enough quotes from recent publications to prove the point.
It's much easier just to point out that they have explicitly acknowledged the time constraint completely outside of the context of the "Generation":
"Especially beginning in 1935, when the identity of the “great multitude,” or “great crowd,” was clearly understood, large numbers of these began to manifest themselves. At first there were hundreds, then thousands, later hundreds of thousands, and now there are millions spread around the globe. God’s infallible Word depicts this group as ‘coming out of the great tribulation,’ being survivors of it, living right on into God’s New Order without ever having to die. (Revelation 7:9, 10, 14; John 11:26) The early members of this group are now in their 60’s or 70’s or older. Jehovah did not allow the ingathering of this group to begin too soon. The “great crowd,” including many of the earliest members thereof, will survive into the “new earth.”" (Survival Into A New Earth, p. 185)
Note that this is strictly a pragmatic consequence of their current understanding of Revelation 7. John's perspective in the vision is at some point in the far future, looking back. He did not "See" a group of people hoping to survive. He saw a large group who as individuals already had survived that event.
If someone is of that group, then they survive the Great Tribulation. If someone is a "Prospective" member of that group, then they at least have to live to see it because you have absolutely no prospect of surviving an event you will never live to see.
Therefore while the idea that the Great Crowd only survives as a class might seem like a tempting explanation from a JW perspective, their interpretation of the vision precludes it. This time constraint is the reason why the "Great Crowd" could not have existed in the year 1700 for example.
(Of course, JW's are quite capable of changing all this overnight...)