Here's the thing though... Thomas and Alito aren't going willfully under a Democratic President, just will not happen.
That would throw the court from 6-3 Conservative back to 5-4 Liberal.
But even if that did happen, the next two oldest are Sotomayor and Roberts, one Liberal and one fairly reliable swing vote.
So we can't just maintain a Democratic president through Thomas an Alito, which is likely to be at least the next two Presidential terms (24-28, 28-32), but beyond that to possibly '36 or even '40.
She does have Type 1 Diabetes though, which unfortunately means an average mortality of 10 years earlier than someone without it. Of course, that's just an average.
We can argue actuarial tables all day, but the point is that replacing Sotomayor with a younger liberal justice has zero downside. It's as close to a free lunch as one can get in politics.
I mean, they do have the best health care you can get. Look at someone like Sandra Day O'Connor, lived to 93. Fortunately she stepped down in 2006 when she was 76.