First, Wren was a key part of the organization when that talent was acquired, so you can't just wave away the success he had so blithely.
Second, John Schuerholz and Bobby Cox, granting their many and manifest accomplishments, are not the guys who need to be judging anyone on a lack of postseason success. Wren was the GM for seven seasons and won 0 postseason series, which is not good. On the other hand, in the last seven years of Schuerholz's tenure we won...one postseason series, in 2001, whereupon we got eviscerated by the Diamondbacks in the LCS. Hurray?
And if you figure that Wren probably shouldn't be blamed too much for the 08 team, you're talking a six-year run where the team was really "his." And in the last six years of Schuerholz's tenure (02-07), we won...zero postseason series. And yet Schuerholz is still trusted to hire and fire people.
Third, even if we grant that Wren was the slavering monster and paste-eating dunce he's been portrayed as, it's not as though he took over the organization through a coup d'etat. He was Schuerholz's hand-picked successor. So what does that say about Schuerholz's ability to evaluate executive talent, and why does he get a chance to hire a second GM when Wren wasn't given the chance to hire a second manager?