It’s not packing the Court. It’s the threat of packing the Court.
With RBG’s passing the likelihood of either Alito or Thomas retiring now to let Trump replace them is almost non-existent. Obviously if Trump wins, we’re permanently screwed and none of this matters. If however Trump loses and the Senate switches party control, then Biden can have a conversation with both Alito and Thomas and let them know the SC will be expanded to 13 unless they retire immediately.
Their choice would be to spend the next several years in a 7-6 minority or a comfortable retirement with nice pension. Neither are young men. Why spend several of your remaining years frustrated in the minority? Under that argument, he might even draw in Roberts — though Roberts seems less likely to retire even if in the minority.
Two of them resign and there’s a 5-4 liberal tilt to the court as the death of Scalia should have produced.
Keeping the Court at 9 would be seen as less politically damaging to the Court than expanding it. If they are unwilling to retire, then expansion would be an easy argument.
Note: For those questioning why 13 instead of 15 or 17, expanding the Court will require folks like Joe Manchin to come along for ride as part of also agreeing to remove the filibuster. A 1 vote Court majority aligned to what it should have looked like after Scalia is a much easier sale than trying to massively pack the Court.