It's much easier when the postseason hero is a free agent. David Eckstein was better in 2007 than he was in 2006, and the Cardinals were basically out of ideas at shortstop—sure, sign Cesar Izturis and wait for Pete Kozma to develop—but he was 33, and his deal had run out, and the Cardinals weren't under much pressure to have him play any part of their 2008 rebuilding effort. Larry Walker rapidly became a True Cardinal in 2004 and 2005, but his departure wasn't even the Cardinals' decision to make.
David Freese wasn't even arbitration-eligible when he had the best postseason anybody could possibly have. He was 28, and injury prone, and a hometown hero, and all kinds of other things that make him a remarkably weird player to have around after the World Series has been won.
Now he's 30, and in the middle of his worst season, and the Cardinals have called up Kolten Wong, leaving him looking a lot like a platoon third baseman.
Once he hit that triple, and then that home run, and then that other home run, there was no way David Freese and the Cardinals' time together couldn't end awkwardly. There was just too much time left on the clock. But in the one way that matters, the clock stopped running a long time ago. However Freese's time in St. Louis ends—and I should note that it hasn't yet—nothing after the World Series could change what happened during it.
In any case—the Cardinals are a better team with Kolten Wong than they are without him, but worst-season-ever David Freese is still a league-average hitter whose on-base (.348) and slugging (.386) percentages look a lot like a reasonable projection for Kolten Wong, who's a year removed from his own lost season in Springfield.
The story of this move is built around their bats, the way Freese's has cooled and Wong's has heated up, but most of its value is on softer ground. If Freese's defense has been half as bad, this year, as the metrics suggest, moving Carpenter to his natural position and Wong into the lineup would make a big difference; if Wong and Freese are able to coexist as ersatz platoon partners they'll make a much better hitter than either is alone.
And if Kolten Wong is the future—he is—and Matt Carpenter is as good as he's been—well, maybe—David Freese just doesn't have a starting role on this team in 2014. Bringing Wong up now, if nothing else, condenses what would be a very tense offseason into a couple of days.