I can't see Feinstein retiring...and I will totally vote for literally any Democrat running in the primaries [probably an absentee ballot for that one].
If Feinstein runs, would you vote for her, or for some random unknown Democrat (who has no chance) in the primary? If she runs, there would likely be no serious primary challenger. Feinstein and some Republican would advance to November where the Republican gets clobbered in the general.
I'm not L.D. Smith, but I think it depends entirely on who this random unknown Democrat is.