If your boss retires, you're out of a job. Pretty sure Feinsteins top staffer is Pelosi's niece (maybe daughter?) And I doubt she'd have trouble finding a job, but most have a financial incentive to keep their boss in office for as long as possible.
Direct incentive yes but it's to the overall detriment to their position in life at this point. Aren't Feinstein and Pelosi in solid blue districts? There's little risk to "losing" the seat.
Republicans have already made it clear that they won't agree to seat Feinstein's replacement on the judiciary committee. She can resign and be replaced by another Democrat in the Senate at large, but we will get absolutely zero new federal judges.
She's missed a lot but a huge number of judges have been confirmed, including a lot in the appeals courts. I'd rather have her available 25% of the time rather than a total stop. Of course I'd rather she be able to leave in a dignified retirement but that requires good faith actions by the GOP, so it's just not an option.