I think the DID diagnosis will be recycled to label BPD types who have fallen so deep into identity exploration/confusion that they’ve latched onto “systems”, are in some no man’s land of consciously faking but with somatic “DID symptoms”...
There's already a really high degree of overlap between diagnosed cases of Dissociative Identity Disorder and Borderline Personality Disorder, with some research showing a comorbidity rate as high as 70% in patients with DID. The relative rarity of DID in the general population and the fact that both conditions are usually linked to traumatic events during childhood (BPD
can develop without severe trauma, but it's uncommon) makes it hard to prove causation, but the idea makes intuitive sense.
With BPD, your sense of who you are can change dramatically based on your mood, how you believe others feel about you, or even what you're doing at the moment. There can be consistent patterns within that, though, that you revert to under similar situations. You might see yourself as outgoing and smart when you're in a good mood, loving and intimate when you're curled up on the couch with a romantic partner, or dependent and helpless when you're convinced that others look down on you. How dramatic that is, and the level of consistency that your self-image has in similar situations at different times, is going to vary based on both symptom severity and changing circumstances. It's not uncommon, though, to feel like you
metaphorically become "someone else". Other symptoms, like irritability and impulsivity, can also contribute to that feeling by causing you to act in ways that you know are self-destructive or harmful to others, and the moderate to severe dissociative episodes common in BPD can be bad enough that you feel like you're watching yourself act out the role of a lunatic in a play. Nothing seems real, some of your thoughts don't feel like your own (even if you know that they are), and it's hard as hell to pull yourself out of it unless you've learned how to manage it through professional mental health treatment.
The need for a core sense of identity isn't really something that most people think about often, because they don't have to. It's only when they've gone through a life-altering crisis, or done something so terrible that it clashes with their sense of self, that an average person feels that sort of emptiness. If a person's interests, beliefs, and character traits were the planets in a solar system, BPD would be the result of interrupted stellar formation. Instead of having a star at the center to illuminate and warm the rest of it, you have a brown dwarf whose gravity sometimes isn't even strong enough to hold onto the cold, dark balls of ice and rock orbiting around it. Obviously a place like that is going to be completely lifeless and dull unless you can find something else to give it meaning. That's usually a close interpersonal relationship, which leads to the deeply unhealthy cycle of infatuation, terrified clinging, and eventual disillusionment most commonly associated with the disorder. It can be something else, though, like a career, community, hobby, or even a concept (especially religion or politics). The key commonality that all these things share is that they offer a delimited role that gives life an externally validated narrative meaning.
This is where Münchausen's comes in. Most people with BPD don't have Münchausen's, but many people with Münchausen's at least have Borderline traits. The sick role comes with a narrative (going from illness to a distant possible recovery) supported by others, who also give the patient sympathy and nurturance. For someone with BPD, the beliefs of others grant subjective truth to a narrative, making things "real" even if they're consciously aware that they're lying about the past, pretending to hold beliefs with no conviction, or in this case faking symptoms of an illness. DID, in particular, would be an easy role to fall into, since there's
some truth to the idea that they feel like several different people. Someone doing this wouldn't actually lack a sense of continuous personhood or experience amnesia while acting as "alters", and they would be knowingly lying about many of their symptoms, but their "alters" would be real in the sense that they were differing states of identity existing in different circumstances. This obviously doesn't
make the claim objectively true or even mean that the person would believe it, but there's a certain internal suspension of disbelief that allows the person to let the role fill the void where an identity grounded in reality
should go.
In cases where they manage to simulate DID well enough to convince a mental health professional, they might even come to the conclusion that what they're experiencing really is DID. Obviously most of them aren't going to manage that, though.