Here's an example if the court is packed. Let's say a state's rights case comes up. The Supreme Court is the court that has to finalize disagreements between the Federal government and the states. These are state sovereign issues. The court is packed by the Democrats. But the issue comes from a more Republican state, let's say Wyoming. The Supreme Court rules against the state. Wyoming complains that the packing of the court caused them to rule against them and they refuse to honor the decision and continue to do what they want. That now puts the Federal government in direct conflict with that state, just like our own Civil War of 1861 when the south thumbed their nose at the north.
What is the federal government going to do? Invade Wyoming? Wyoming's governor calls up its military reserves and constripts its residents with their guns preserved under the 2nd Amendment to defend the state. They put militia at their border and tells the Federal government to stay out. Is a president going to use Federal forces to invade Wyoming to force the Supreme Court's judgment? That would only work if the court's decisions are respected. But that's gone. No one respects it anymore. South Dakota sends its militia to Wyoming to help its neighbor. The president decides to back off, even a Democrat one, much less a Republican president. The court's ruling has no weight. Everyone laughs at it. The court is destroyed because of the packing.
Now imagine if a packed court reverses earlier court decisions regarding the 2nd Amendment and the right o bare arms. Do you thinks millions of Americans are going to turn in their guns? How many governors will just ignore the ruling just like states have ignored Federal marijuana laws. We'll become a nation of men not of law.