I am not a psychiatrist, but as a pharmacist and patient with mental illness, nothing has been more vital to my recovery than working. I'm not currently diagnosed with bpd or any personality disorder, but I do have bipolar disorder and a psychotic disorder. When i was on disability, it didn't further my recovery - I rarely got out of bed and lived in horrible conditions because I never showered or cleaned. Sure, it was hard to get out of bed before I was on disability - but once my claim was approved, there was zero incentive to keep pushing myself to try.
I do think disability can be helpful in the short term to allow for a progressive return to full time work because the transition is extremely challenging emotionally and cognitively. Being on disability can also open up employment support services that wouldn't otherwise be available. I can also see a need for long term disability in patients with truly severe impairments. However, in my (non-expert) opinion, patients who truly cannot engage in competitive employment would still benefit strongly from regular engagement in volunteer work. I just don't see how anything good can come of never leaving your home or even getting out of bed. I think the pandemic has shown us how vital regular social interactions and predictable routines are to our well-being. I do have negative symptoms with my psychotic illness, and they make it extremely hard to care for myself - but disability takes away any incentive to even try.
I genuinely don't intend to minimize the struggles of people with mental illness. However, those struggles don't mean we should put more people on long-term disability - they mean that our society should be willing to provide intensive hands-on employment support so that people can succeed despite their illness. Long term disability doesn't solve the problem of severe mental illness; it just transfers it to the shoulders of the government. I'm not a policy expert, but I think government-sponsored financial incentives to hire people with disabilities benefit everyone. The employer gets compensated for any extra support they need to provide, the person with a disability earns more money than they would have on SSI/SSDI and reaps the many mental health benefits of working, and the government can collect taxes from the patient since they are working.
TL;DR - I am nowhere near an expert, but personal experience has shown me that there's nothing more healing than work. Rather than expanding SSI/SSDI, we should put the money toward employment support services that make it possible for everyone to contribute to society.