“I agree. But what are the chances that the 1869 Judiciary Act would be repealed with a new one? And would the existing Supreme Court have the authority to prevent that from happening?”
If the Democrats get the Senate, the House and the Presidency, and are willing to nuke the filibuster (which they have already said the will do), they can change it with a simple majority vote in both Houses. I can’t think of any grounds for SCOTUS to prevent it.
One way it could be done would be if the court declares the replacement of the Judiciary Act as unconstitutional — say, because it considers packing the court would put too much of a burden on its duties under Article III of the Constitution. Who'd be in a position to say otherwise? After all, the law against abortion was declared unconstitutional on privacy grounds, even though the Constitution says nothing about privacy,