As Michael Cohen points out in this piece, he's hardly the first lib to tell the aging jurists that it's time to go. President Obama has made two appointments to the High Court, Justices Sotomayor and Kagan. Neither changed the balance of the court. Liberals are concerned that if Ginsburg and Breyer are still on the court after 2016, a Republican president could very well replace them and that would be a significant game changer.
Right now the Obama legacy is maintaining the status quo on SCOTUS. President George W. Bush's legacy was supposed to be Roberts/Alito, but some big Roberts' rulings have that in question. If Bader-Ginsburg and Breyer are still on the court November 2016, it will become one of the most important presidential elections in a long time.