From a really solid, comprehensive piece about Mariano Rivera's past and future, via Tyler Kepner in the Times:
Nothing could change his decision, Rivera said then, without revealing what it was. But retirement seemed the clear option, and when asked by reporters Thursday night if he would ever pitch again, Rivera, 42, said he did not know.
Understand this about Rivera: his career plans are never assured, whatever he says. After the 1999 season, Rivera told the congregation at a church service in his native Panama that he would retire after four more years to become a preacher. A decade later, at Yankee Stadium in the afterglow of the 2009 title, he said he wanted to pitch five more years.
How different would history look if Rivera had become a full-time preacher after the 2003 season?
Well of course that's difficult to say. The Yankees have won just one World's Championship since then, in 2009. The Yankees swept the Twins in their Division Series, topped the Angels in six games in the ALCS and beat the Phillies in six in the Serious. Rivera did pitch multiple innings in two close games, one apiece in the ALCS and the World Series. So it's possible that if Rivera had retired after 2003, the Yankees' World's Championship-less string would today run all the way back to 2000, which would be odd.
But as with everything else like this, it's really impossible to know what would have happened differently. Maybe the Yankees would have signed some big free-agent closer, who wouldn't have signed with another team and propelled that team to a championship. Now that we've lived through it, it's just hard to imagine a world in which Mariano Rivera didn't pitch for all those seasons with the Yankees.
Meanwhile, tacking on five seasons after the 2009 World Series would have meant a career stretching through 2014, during which Rivera will be 44 years old.
Realistic? Well, there's never been a 44-year-old closer. But even leaving aside knuckleballers and spitballers and other strange -ballers, six 44-year-old pitchers have thrown at least 150 innings in a season.
Kepner's right: We really can't begin to know what Mariano Rivera does until he actually does it. I just won't be surprised if he does pitch again.