The Undertaker has either defeated or had highly competitive matches with every single big money-drawing wrestler that has been employed by WWE in the Vince McMahon, Jr. era of running WWE. From Hulk Hogan to CM Punk, he's literally faced them all. It can be argued that there are no more big money draws in WWE left for The Undertaker to face, and in saying that, it's time for him to retire. WWE being in an era where they're regenerating themselves possibly allows for the company to allow a tireless contributor like The Undertaker to finally (and deservedly) leave for good.