Well, in a word—exactly. My example was meant to illustrate what happens if Feinstein's family tries to "step up" (as someone posted above) but she isn't onboard. They really can't do some kind of end-run around official procedure to nudge her out while saving her dignity. Assuming she's not declared legally incompetent, SHE has to be the one to write (or at least sign) that letter of resignation... and how do you get her to do that if she still firmly believes (in her sadly addled mind) that she's capable and has no desire to step down?

That's why this is all a lot more complicated than "how could her family let her publicly disintegrate like this (while also depriving her constituents of a competent senator)?" The bottom line is that if she won't be convinced, there's nothing they can actually DO—short of having her declared incompetent, as I've discussed.