Couple of other points;
In the case of Melendez, who was Strikeforce Champ - all of his opponents he beat to become SF Champ are typically UFC wannabes, meaning they are guys not good enough to be in the UFC, or were released by the UFC. So an argument can be made where all of UFCs LW fighters are better than the vast majority of SF LWs. That is not the case with Melendez himself, but is the case for the bulk of the opponents he beat. Does he deserve the immediate shot? Is Bendo going to give him a very rude awakening?
If a Champ goes to another division to fight, you can kiss any title shots goodbye in his original weight class for at least 6 months. He'll need a minimum of 3 months to prepare for other weight class fight, then recovery time, and then start camp for the next fight. If there is a truly deserving contender in the wait, he is fucked for over half a year.