When I first read 1984 I also thought like you, that this was just Orwell writing an essay about his invention, but there are some passages in this essay that show it's being written from an "in universe" perspective, i.e. by someone who lives in the same universe that 1984 "happened".
Just an example:
> "Various writers, such as Shakespeare, Milton, Swift, Byron, Dickens, and some others were therefore in process of translation [to Newspeak]: when the task had been completed, their original writings, with all else that survived of the literature of the past, would be destroyed. These translations were a slow and difficult business, and it was not expected that they would be finished before the first or second decade of the twenty-first century."
There are many tell-tale phrases there, but to pick an example: "it was not expected that they would be finished before [...]". "Expected" by whom? This means very little if it's just Orwell saying so, and it makes more sense if our fictional narrator is actually describing the history of his/her world.
Once you accept this, you must also accept Newspeak failed, which in turn hints at Ingsoc being defeated.
That is a lot of interpretation based on fairly implicit textual evidence. I am not disagreeing with your interpretation, only pushing back that this is the only possible interpretation.
Alternate interpretations exist. Including:
1. Orwell's writing slipping between his voice and the "narrator in the scene"
2. General stylistic choice to follow scientific writings style guides - 3rd party, impersonal, etc.
3. General de-personalization of the voice of the System, so as to be more fearful
I'm sure there are many others I have missed. Again, my point is just that many valid interpretations exist simultaneously - so "once you accept this" is not the fait accompli that I interpreted from your comment.
I agree alternate interpretations are valid -- I mentioned I used to believe the same as the commenter I was replying to -- just unlikely.
Given Orwell's preoccupation with language, his mixing authorial with fictional voice in this way would be too clumsy.
There's also the fact another fictional essay exists in 1984, namely the one supposedly written by Goldstein "explaining" the nature of power and the status quo. Do note this account was written, in the fiction of 1984, before the complete success of Ingsoc, and once Newspeak was fully implemented it would have been neither possible to write nor needed.
I don't deny other interpretations are possible, but I think this essay works much like Lord of the Rings' many appendices: they are describing a piece of past "history" as if it was real, they are not the voice of an author from our world describing a fake world.
Agreed, given that this is fiction, everything is possible, however...
... since this is an in-universe account of Newspeak (because of the way the essay refers to Newspeak, we know it's not just Orwell the author speaking), and since this account wouldn't be possible in Newspeak itself, and furthermore, and because this is written in past tense, we have a pretty good indication that Newspeak failed. And because Newspeak and Ingsoc are irrevocably married, we also have a good indication that Ingsoc itself must have failed.
This is a coda explaining in-universe some details of a failed regime, much like the similar one at the end of The Handmaid's Tale (though that one is way more explicit).
Just an example:
> "Various writers, such as Shakespeare, Milton, Swift, Byron, Dickens, and some others were therefore in process of translation [to Newspeak]: when the task had been completed, their original writings, with all else that survived of the literature of the past, would be destroyed. These translations were a slow and difficult business, and it was not expected that they would be finished before the first or second decade of the twenty-first century."
There are many tell-tale phrases there, but to pick an example: "it was not expected that they would be finished before [...]". "Expected" by whom? This means very little if it's just Orwell saying so, and it makes more sense if our fictional narrator is actually describing the history of his/her world.
Once you accept this, you must also accept Newspeak failed, which in turn hints at Ingsoc being defeated.