I'll agree, with the caveat that perhaps if the BPD person admits they have BPD, is in therapy, and is trying to get better... sticking around may be OK.
But it's no different than when someone has alcoholism or a drug habit. If they don't want to get better, get out.
This is the only hope, but I'm afraid one would have to put limits on the amount of time spent in a relationship and do some reflecting on their own life and goals. Realistically, is one willing to spend decades with a BPD in therapy hoping for good results? I'm just trying to guard against false hope.
But it's no different than when someone has alcoholism or a drug habit. If they don't want to get better, get out.