Senators don’t work that much. She had plenty of time with her family. She didn’t retire because she made lots of money being a centrist senator in a safe seat with enough seniority to do whatever she wanted.
This won’t impact the shutdown at all, since the House republicans want a shutdown.
Now Newsom will appoint a moderate black woman who will provide the optics without causing chaos. You might think that sounds cynical, but that’s exactly what Newsom already promised to do.