Coetzee's Elizabeth Costello device

I am working my way through Coetzee's novels in no particular order, and I have just finished Slow Man. Elizabeth Costello's role becomes clear there. Coetzee, no doubt all too aware of his self-centredness; in fact he keeps alluding to it through his secondary characters, who keep asking if they really can be considered secondary, if anyone can; so as I was saying, to deal with that dilemma, he creates this other author's voice, to take him outside J.M. In a sense it works. He manages to see things differently, and I am sure the choice of gender helps him. Empathy is strong in his writing, but it is a clinical empathy, one rooted in observation, like a photographer that sees misery, captures it and then moves on. It is not care, it is halfway to care. So The Elizabeth Costello device pushes the plot towards action, towards mistakes, she forces him to do and to confront.
Again, why the fuss? Why not just write from the perspective of other voices than his own? Probably because he is so honest and is not wired that way.
The method succeeds in making his work interesting, but as John Crace in his pastiche of Coetzze in the Guardian  notes, he keeps writing the same book. Why not?

In fact I have argued the other way for songwriters. Is the medium a factor? Can songs be egocentric, since they are an accepted form of wooing? Are novels different? No answers here.