The Rock of 2012 is sure as hell different but that's what you call evolution. Rock's promos as stated above started to change from 2002 onwards, mainly as the Attitude era ended. His promos changed, left in 2004, came back in 2012 and his promos seem to be more intune with Rock as a person and his sense of humour than his cocky, brash Rock of the past decade.
You think Taker would still be around if he had sticked with his Deadman gimmick of 90s? Nope. He's kept going because he sensed he had to change somewhere down the line, he did that by changing to the ABA from 2000-2003.
Bottom line is, Rock is still entertaining and keeping the fans on their feet.
Unfortunately with Cena, had a successful rap gimmick, which he tried to move away from - but sadly didn't change to anything fresh or new. He turned his gimmick to the Never back down, never quit but that just came across as cheesy. Maybe not his fault as the writers/Vince wanted him to be the man but Cena's not evolved his character successfully, hence the boos. Maybe a heel turn down the line could do something for him.