There have been 9 court justices for 150 years. Why do the democrats now think they need more?
Ok. Nobody appears to have any better reasons than “but muh historical precedent,” so I’ll answer your question now.
I don’t agree with court packing, but to me the most compelling argument in its favor is that expanding the court is a way to preserve the status quo.
That status quo being that we presently have a court that does not go around dramatically upsetting established precedent based on political ideology. (See
June Medical Services example I gave earlier. Roberts (and Kagan) understand that a great deal of the court’s authority is derived from the rule of law, which is best served when the outcome does not appear to be rooted in partisan politics.
Maintaining that balance requires some degree of deference to and respect for precedent, even in the face of jurisprudential disagreements about whether the correct outcome was achieved. In practice, that means incremental changes rather than sweeping away the entire structure.
Coming to your answer: the impetus here is that the confirmation of Amy Coney Barrett, as a replacement for a Ruth Bader Ginsburg, threatens to upset that status quo. Not only because she shifts the composition of the court significantly, but because she has written law review articles attempting to reconcile overturning some precedent that is inconsistent with her originalist philosophy, while retaining some other inconsistent precedent.
So, one could argue that what Republicans have done, between Scalia’s seat and Ginsburg’s, has unsettled the aforementioned status quo. They now seek to freeze it at this new anticipated normal by shaming democrats with the same extra-constitutional restraints on congressional authority, such as prudence and respect for established norms, that they themselves rejected in 2016. (For proof of concept, see: this entire thread).
So, to whatever extent that the present status quo has value, a case could be made for expanding the court to 11 and returning the swing vote to Roberts, who has used it responsibly since Kennedy’s retirement.
Beyond 11, the case gets weaker, but I’ve yet to see an argument against it that is anything other than partisan hypocrisy.