How does Atticus respond to Alexandria's suggestion that they don't need Cal anymore?

From the text:

Atticus’s voice was even: “Alexandra, Calpurnia’s not leaving this house until she wants to. You may think otherwise, but I couldn’t have got along without her all these years. She’s a faithful member of this family and you’ll simply have to accept things the way they are. Besides, sister, I don’t want you working your head off for us—you’ve no reason to do that. We still need Cal as much as we ever did.”
“But Atticus—”
“Besides, I don’t think the children’ve suffered one bit from her having brought them up. If anything, she’s been harder on them in some ways than a mother would have been... she’s never let them get away with anything, she’s never indulged them the way most colored nurses do. She tried to bring them up according to her lights, and Cal’s lights are pretty good—and another thing, the children love her.”