I believe her chief aide is one of Pelosi's children, and people read into that manipulation by Nancy herself -- usually along the lines of "she's propping up Feinstein to keep out a progressive replacement!"
Feinstein belongs in a memory care ward rather than the Senate, but it doesn't take nefarious scheming by everyone's favorite bogeywoman to explain a cantankerous dementia patient refusing to accept their limitations and step away from something that they find comfortable and familiar.
To play devil's advocate, when Social Security was established (bringing with it the concept of a "retirement age"), the age of eligibility was deliberately set such that less than half of Americans would live long enough to draw on it. The clear expectation was that you would work until you couldn't anymore.
That said, in an era when changes in life expectancy are starting to take on a K-shaped distribution and labor force participation has been on a long steady decline, tying governmental income support to age and employment duration is becoming distributionally regressive. I'd much rather have some sort of UBI system that everyone can benefit from.