Earlier quoted context omitted.
From everything I could tell, one of the reasons Sen Feinstein was sticking around was that the GOP had indicated it would not allow Senate Democrats to replace her on the Judiciary committee. This would deadlock the committee, effectively stalling all judicial appointments and confirmations. That was in the case of a retirement. This is a death and reappointment. I am unsure if the new Senator would assume Sen Feins…
I think this is a bit off. I don't think the GOP in the Senate can stop California from appointing Feinstein's replacement, or keep them from being admitted to the Senate. They can keep the replacement from being appointed to the Judiciary Committee, but that is not an inherent right for senators from California.
California can appoint an interim Senator (per their constitutional & laws - I believe the governor does it without any approval from other parties).
Committee appointment is by Senate rules. There's no automatic right to a seat on a committee. Judiciary is a massively important committee because it oversees the approval of federal judges (without committee approval, the nominees never get to the floor for a vote).
The GOP can block a committee replacement via filibuster. The Democrats could attempt a rules package to prevent filibuster, but Manchin (D-WV) has historically been against those.