Justices tend to try to avoid retiring when they think they will be replaced by those who would undo their decisions.
Many folks assume that a new Justice will immediately cause the reversal of the Heller
decisions. That's simply not the case.
First off, the Court has to be called upon to make a decision. They don't just hand down opinions: there has to be a case requiring the resolution of a constitutional question. I'm not exactly sure what kind of case would lead to a challenge of Heller
Second, the Court has to be careful not to reverse itself in short order. Doing so casts their authority into doubt. They reverse themselves slowly, if at all. Consider the 60-year gap between Plessy
and Brown v. Board of Education
Now, can they prevent further progress forward? Yep. But I wouldn't worry about anything going south in the near future.