Are you basing that on a reading of the appendix, or just on the sentence I excerpted? It's not really clear from the sentence itself, but in the context of the paragraph, it makes a lot of sense.
Pay close attention to what's cast in the subjunctive throughout the appendix, and this reading, or at least the ambiguity, seems intended.
Another good example is "In 1984 [the year, not the book], when Oldspeak was still the normal means of communication, the danger theoretically existed that in using Newspeak words one might remember their original meanings. In practice it was not difficult for any person well grounded in doublethink to avoid doing this, but within a couple of generations even the possibility of such a lapse would have vanished." The consistency seems more than accidental.
It has been many years since I read 1984 or its appendix, so I was basing what I said mostly on my recollection of it and the sentence you quoted. But having just now rescanned the appendix, I think it is written from the perspective of someone in the real world where George Orwell and the novel exist, not someone from the future in the fictional world of the novel. See for instance the sentence
> Newspeak was founded on the English language as we now know it, though many Newspeak sentences, even when not containing newly-created words, would be barely intelligible to an English-speaker of our own day.
I presume that "our own day" refers to real-world 1948 when the book was written. If my interpretation is correct, it explains the usage of the past tense, which is often how we refer to fictional worlds in books or movies, even if these fictional worlds are set in the future. We would say "Captain Picard lived many years after Captain Kirk," even though both characters are set centuries in the future.
That's how I originally read it, and I think that's a valid reading. I think Orwell probably wanted readers to get that appearance. But the use of the subjunctive "would have" suggests--though doesn't prove--something more.
Your example explains the past tense but not the subjunctive mood. For example, would one say "Captain Picard would have lived many years after Captain Kirk"? To use the construction "would have" in this way casts the event as something that could have happened, but didn't. Similarly, Orwell's constructions paint the events after the publishing of the "perfected" Eleventh Edition, some time after 1984, as events that were planned but that never took place. If it were just anticipation of a future event that wasn't yet in the fictional timeline, I would expect "would" or "may have".
It's ambiguous, but it seems to me like he went out of his way to leave both readings open.
> For example, would one say "Captain Picard would have lived many years after Captain Kirk"?
Not for that specific example, but one might certainly speculate "Picard would have retired to his ranch in Marin County after retiring from Starfleet" (I'm guessing the later years of Picard's life have been covered by the fiction, but let's pretend like the series just ended and we're speculating about what the Picard character's later years were like.)
All this having been said, I do think there is supposed to be some ambiguity, or at least some missing information, about what happened to Ingsoc.
Pay close attention to what's cast in the subjunctive throughout the appendix, and this reading, or at least the ambiguity, seems intended.
Another good example is "In 1984 [the year, not the book], when Oldspeak was still the normal means of communication, the danger theoretically existed that in using Newspeak words one might remember their original meanings. In practice it was not difficult for any person well grounded in doublethink to avoid doing this, but within a couple of generations even the possibility of such a lapse would have vanished." The consistency seems more than accidental.