> What makes that especially pernicious is that, tracing all the way back to the era of Freud, BPD has been viewed as essentially untreatable
I've got the impression that BPD, unlike other personality disorders (or things labelled as such), isn't actually considered essentially untreatable, at least not nowadays. BPD probably has a notorious reputation, possibly because of the potentially emotionally taxing nature of living with or treating emotionally unstable people, and perhaps because it could be difficult to treat. But no modern factual source I've come across has considered it untreatable.
Even a superficial source such as Wikipedia says the majority of those who have been diagnosed achieve recovery over time [1]. I've also read from reputable sources [2] that in 5 or 10 years the majority of patients no longer fulfil the diagnostic criteria.
That doesn't invalidate what you're saying, of course. The symptoms do overlap with a lot of other disorders (which is also true of many other psychiatric diagnoses, which is why differential diagnosis is important). And the very fact that just about every other thing that's called a personality disorder is considered nearly untreatable (and stable over time), while BPD isn't, is probably a reason to consider whether it should belong in the same category.
[1] https://en.wikipedia.org/wiki/Borderline_personality_disorde...
[2] the ones I remember are non-English but I haven't got the impression that they differ from Western mainstream