Feinstein was a mess during the Kavanaugh hearings.
That was when the public at large realized just how far she had declined. Same for R's with Grassley, who is the same age, but he was nowhere near as bad as DiFi. Grassley resigned his chairmanship over it, as everyone saw he was no longer up to the task. For some reason, Grassley has stayed pretty stable while Feinstein remained on a downward trajectory until her death. It seems Grassley also intends to die in office, trying to beat Strom Thurmond's record as the oldest Senator.
At this point, I'd support a constitutional amendment to set a maximum age for all federal offices, elected and appointed. Or term limits. Either would greatly reduce the odds of these senile idiots deciding our future.