One thing that's smart about Joe Biden shifting the debate from the number of justice on SCOTUS to the _entirety_of_the_federal_judiciary is that Senate Republicans have over the years submitted many bills to reshape the circuit courts. See, e.g., https://www.courthousenews.com/idaho-republican-launches-bid-to-split-9th-circuit/

Splitting up the 9th Circuit has always been the Idaho Senate delegation's white whale.

Senate Democrats can say to Mike Crapo and James Risch Senators, "OK, you want to split up the Ninth Circuit? I'll raise you the 5th, 6th, 7th, 8th, 10th, and 11th Circuits."

I mean, we can quibble all day about the merits of a 9-Justice SCOTUS. But can anyone say with a straight face that the geographical boundaries of the federal appellate courts, and their numbers, are sacrosanct?

Surely these can be reshuffled every generation or so.