Di Matteo wasn't a rookie. Also, he is a shit manager, if that makes any difference. Thirdly, KO competition is different, and I think we all know them winning was a bit of a fluke, and that's me being kind.
Pirlo might be a good or even great coach one day, but that really would be absolute desperation and a serious swallowing of pride and image to make such a change. It would look like a really shitty move from us, and admission of failure.
Anyway, I should never say never, modern football is so bizarre.