Lot of credit here goes to Katie Porter for breaking the barrier where no real leading California politician would dare challenge her. When it was clear to Feinstein’s backers, since I doubt she even really understands the situation at this point, that she was going to have to run in a real primary next year, they naturally backed away.
Also, Dianne Feinstein has been a terrible senator from Day 1. Literally any Democrat from California will be better.