It's absurd to think that with the retreat of the federal government, criminals will turn into saints; you have to confront them with authority.Calderón's characterization of his opponents is a bit exaggerated, but this gets at an important question about Mexican drug policy, one that has lots of implications about the future: to what extent is the violence of the past four years caused by Calderón's aggressive stance? Relatedly, if the next president adopts a much softer approach, will violence automatically drop?
The explosion in narco-killings certainly coincided with Calderón's term, but it was on the rise toward the end of Fox's term as well. Lots of other countries have seen explosions in criminal violence that were unaccompanied by a suddenly revamped federal enforcement drive; it's not like deploying a firm hand is a prerequisite for a spike in organized crime. It's possible that the deployment of the army was the magic ingredient for a more violent Mexico, and lots of people seem to think that it is (including Calderón himself, who always points to the violence as evidence of his medicine's effect), but it's certainly not proven. I think it is more difficult for analysts and politicians to confront a situation without any single coherent, logical explanation than it is to agree on a simple cause and effect (despite the fact that the correlation hasn't been proven), and then just argue about what that effect tells us.
Anyway, if you could demonstrate that Calderón's crime policy was the single most important driver of the violence, and that internal dynamics of the drug trafficking industry were of secondary importance, then you certainly could make a case for the next president calling off the dogs and giving everyone a break. But if you're not sure, what's a politician to gain from a less aggressive approach?