Even Del Piero would have joined Milan in his shoes. Del Piero was so faithful for many reasons the biggest of which is that he is a classy man but up to 2006 he was in a very comfortable position that not a lot of people would choice to opt out of. And in 2006 he was already such a symbol and not in his prime anymore so the choice really wasn't that hard for him.
The biggest sacrifice in 2006 came from Buffon, Nedved and Trezeguet in that order. It was extremely hard to do, that's why I can't blame Cannavaro. But because of his decision he can never be in the same conversation as the loyal 5.