Granting that 1) Republicans don't fight and 2) Democrats are united on the court-packing legislation, is it likely to pass?
That's not the point.
The REAL objective here is to scare the bejabbers out of Roberts (an easy task) and the other "conservatives" on the Court. This was the objective of Roosevelt, too.
Because then the Court will become far more compliant, just as it was under FDR after his threat--despite the fact that it was blown off even by his own Party.
If you don't believe that, look at Roberts' decisions since Trump assumed office. He's cowering in dread, peeing his pants, and begging the Democrats to "respect me," while shredding the Constitution.
That's not working out well, is it?