The moment that a progressive majority on a newly expanded Supreme Court ruled decisively on any of these issues (or on any of those conservatives care most about, including gun rights, religious freedom, and economic liberty), red states would simply disregard the opinion as illegitimate, nullifying it on the grounds that the decision was made possible by an explicit act of cheating.
That Pyrrhic victory would then be followed by an outright rout the moment Republicans retook control of the White House and Senate (perhaps riding, in part, a backlash to court packing), a position from which they could pack the court with even more conservatives to surpass the number of newly added progressives. By this point, no one on either side would feel any obligation at all to defer to the authority of the nation’s highest court. Every one of the disputes in our politics that the Supreme Court had if not settled then at least placed in abeyance would surge back into the political arena at the local, state, and federal levels, with no authority capable of adjudicating the clashes.
Down that path lies political breakdown and ungovernability.