Absolutely. I can't help but think that Kate's royal life would be miserable, if it even existed, had Diana not jolted the Royal Family into becoming more in touch with changes in society, feeling empathy, and being a kinder institution in general. I mean, Diana had to take a virginity test. Yeah that was a few decades ago, but it wasn't that long ago.The bigger difference to me would have been with the Queen and the rest of the Royal Family. I think Diana's death really changed them.
I'm not sure how things might have developed if she had lived. Assuming that Diana did have BPD, I would think that treatment options have improved. Also, she might have found satisfaction (or attention) in her charity work. I don't think she would have stayed static, any more than anyone else has.
As with all mental health treatments, some people can be treated more effectively than others. There are newer medications that have been effective for a larger percent of patients.Treatment options have improved, but getting someone with BPD to (1) get diagnosed and (2) get treatment remains difficult. Apparently, most people with BPD get better. Those who don't get better, however, tend to experience greater social dysfunction, more hospitalizations, and increased feelings of chronic emptiness (regardless of how full their lives may seem to outsiders).
Regardless of whether we assume that she had BPD, any statement of what her life would have been like in her forties and fifties is wild speculation.
I'm not sure Diana could have been effectively treated, however. Mental illnesses were and are highly stigmatized, and I bet the stigma was particularly strong inside the Royal Family institution, which is much larger than just the family members. Given her role, would she have been able to take those medications with all their side effects (weight gain, fatigue, foggy mind, slurred speech, etc) without encountering public ridicule and criticism? Even if she could, I doubt she would have gotten better. That was about the worst environment possible for a mentally ill person. A therapeutic mental health environment involves privacy, the opportunity to express your negative feelings (crying, wanting to be alone, not wanting to go outside), and most importantly, trust. You need to trust that those around you support you, will protect your confidentiality, care about your well-being per se rather than for the sake of their own desires, their own image, or the image of the institutions they represent. You need to trust that they like the real flawed complicated you rather than the perfect public you, and they want to be with you because they like you as opposed to your status and popularity.
Diana had none of that. She had no privacy, had her trust betrayed, had a spouse and powerful in-law family that shunned and humiliated her, was bombarded with extreme opposite messages (You're the best! You're the worst! You were the best yesterday but today you're the worst and tomorrow you'll be the best again!), and was expected to always suppress her negative emotions. She could not trust anyone to keep things confidential. She could never know who was genuine, what words were genuine. She knew that as many supporters as she had, she had an equal number of haters--and neither group even knew her! She also knew that negative sensational news sells more than positive news, and when it comes to British tabloids, they follow the money.
Within the Royal Family institution, Diana spoke a different language. She was outwardly expressive and emotional whereas those around her used a secret code, honed from within over centuries, in which members say more with silence, shunning, oblique references, and backhanded compliments than with clear words.
As someone who has struggled with severe mental illness, I rue that when Diana is remembered, few people acknowledge the enormous pain she suffered from her mental illnesses. They gloss over that. She had to suffer in order for future royals to have a better experience, and she never lived to see that because it took her death to motivate those changes.
In a way, even after her death, people have chosen to ignore her cries for help.