Biden is 80 years old, and Feinstein is 90. The numbers aside, I see no resemblance in their health statuses. I'm not discounting that voters are indeed "squeamish" about Biden's age, but we hear that he takes good care of his health. Also, a senator's term is 6 long years so Feinstein, at age 85, should have not run in 2018.This recent episode (and Feinstein) will only serve to make voters even more squeamish about re-electing Biden.
IMO he needs to replace Kamala with a better VP candidate because there's probably a decent chance he won't finish a second term, if he even wins at all. The voters need a better backup plan than VP Harris who may be the only person in the country more unpopular than both Biden AND Trump.
Kamala is a disaster as a national "candidate" or "heir apparent," but she's non-replaceable at this point. If you replaced her with a non Black person, you'd lose some of your most reliable voters. Unless she does something incredibly dumb like taking a Tulsi Gabbard turn, they're not swapping her out. She won't be the Democratic nominee in 2028 either.
There's being squeamish and then there's being irrationally panicked. There is no indication that Joe Biden is on death's door so I don't know why you'd say there's a "decent" chance Harris will be POTUS during a Biden second term.