They are, but only Abbati (who is starter material) has played long with Milan. All other mentioned players are still pretty new Milan players. I still think, they need a leader and I doubt Abbiati is enough. Two of Nesta, Ambrosini and Gattuso should stay IMO. If Nesta has to leave (his own decision), Ambro and Gattuso should stay.
"When Costacurta and Maldini won they didn't celebrate much. They were thinking about winning the next game" - Alessandro Nesta