Creating a Democratic majority on the SCOTUS isn't something that can be realistically done by one POTUS's appointing opportunities. Right now the SCOTUS is 6 to 3 Republican/Conservative majority. The 4 oldest Justices are currently Thomas, 74, Alito, 73, Roberts, 68, who are all Republican, and Sotomayor, 68, who is Democrat. Scalia was 79 when he died. RBG was 87. Breyer retired at 83.. Even assuming that all four of those oldest Justices leave the Court at 83 years of age (which is the average of the last 3 to leave), and assuming Biden is reelected, none of them would leave during his term. Harris, or some other Democrat would have to win after Biden, to have a shot at replacing Thomas and Alito with Democrats.