Why would McCain replace her?
After all, she's a genius irt energy policy. According to him, she "knows more about energy than probably anyone else in the United States of America."
Plus, she's a foreign policy guru. After all, she is the governer of a state right next to Russia!
Besides, he really, really likes her: "She's a partner and a soul-mate."
Maybe she can help keep track of how many houses he owns, or help him locate the Iraq/Pakistan border.