There are four ifs that could lead to the most progressive Supreme Court ever. Two of which are all but certain (McConnell replacing RBG and a Biden win). One is very likely (Dems win the Senate). The other (Schumer's actions) is up in the air.
If the first three happen Schumer would have to follow through with his threats that nothing is off the table. The first thing he should do is add DC and Puerto Rico as states and sit their four Dem Senators. The next step is to expand the Supreme Court to 15 members with a simple majority vote.
Even if the Dems only win 51-49. They would be up to a 55-49 lead with the new states. The 2022 Senate election looks very, very predictable (
https://en.wikipedia.org/wiki/2022_United_States_Senate_elections). At most, it appears that only a few might even be competitive (AZ - Kelly (D), Rubio (R), GA, NV (D), PA (R), WI(R)). That's not enough to swing the Senate.
Add to that, Biden would veto any other expansion bill if somehow the GOP took the Senate while was in office. If Harris on another Dem wins in 2024, it puts any change off the table for another 4-8 years.
During Biden's first term, it's likely he'll replace Breyer. If the Dems hold serve through 2028-32, it's also likely that they will replace Thomas.
By 2028 or 32 at the latest, TX and FL will become purple.
If Schumer and has some balls, he could parlay the McConnell/Trump replacement of RBG into at least a quarter century of a very progressive Supreme Court.