A little JW history....
When the doctrine was first introduced in 1935, it was thought that the "great tribulation" had already started with WWI and that they were in the "eye of the tornado" as it were. (This is how they understood the days being "cut short")
They really, truly thought at the time that the "Jonadabs" had "come out" of the "great tribulation."
When this was changed, many years later, (early 70's) and the "great tribulation" became entirely a future event, all that could truly be said was that persons with the "earthly hope" were prospective members of the "great crowd" (As Onacruse points out.)
However this inseparably tied the "great crowd" doctrine to the old understanding of the "generation" like siamese twins. What had made the identification of these prospective members of the "great crowd" possible in 1935 was the proximity of the end.
To even be a prospective member of the "great crowd" you have to at least live to see it. --Else there is not and never was an "prospect" of surviving it. In other words, if a final generation before the end cannot be identified, then neither can the "great crowd" be identified.