Whoever replaces David Souter would have to be pretty extreme to make any impact on the makeup of the Supreme Court. Nor would the departure of Ruth Bader Ginsburg make any substantial difference at all. So for the short term, there is no likelihood that Obama could make much of an impression.
Unless the appointees are relatively young, and they could remain on the court for a quarter century or longer. And that is where the real mischief lies.
Whoever replaces Justice Gay-O is unlikely to be a mediocrity like he. Something to celebrate. How well he represented and symbolized Big Daddy B.