If Nunn loses, she'll go for Isakson's seat in two years, whether he retires or not.
She won't. Her funds would be exhausted by then, and going up against a well-funded, popular incumbent in an (atlas) Blue state during an election year is much harder than going up against a bruised opponent who just survived a competitive primary. Nunn is likely to lose that latter contest anyway.
Let's say he retires, since he is about the same age as Saxby. That's a fun name. What then?
Part of the reason Saxby retired was because he would have faced a strong primary challenge if he didn't. Isakson, who is much more popular and much more conservative will not face that problem. But if he doesn't, the whole race will be a repeat of 2014's scenario, which no one, especially Isakson, wants. He'll run.