Why did "The Devil in Prada" finally change its supervisor?

The devil wears Prada finally changed the director because:

Originally, Nigel wanted to be James' partner, but because irv, the president, wanted follet, the editor-in-chief of French edition, to be the editor-in-chief of American edition (kicking Miranda out), and Miranda asked follet to be James' partner, Nigel had to stay in runway.

In order to prevent herself from being replaced, Miranda should have given this position to her subordinates who have worked for her for ten years. However, in order to keep this job for a long time, she has left a hand. A whole paper of designers, photographers and models are all her chips. If the director is changed, this magazine will change a lot.