Gen. Petraeus is an extremely convenient replacement, because if there is one US military commander with ample experience in transmitting false hope to the Congress, it is surely him. In fact in his role as Centcom Commander Petraeus has been regularly called upon to predict that the situation will worsen in Afghanistan but that some improbable victory would emerge at some indeterminate point down the road, if only everyone believes hard enough.
Ultimately, there will be a "Saigon Moment" in Kabul, where the US government will need to declare victory, bring its troops home, and negotiate with whatever government is left standing for the oil pipeline and mineral rights.
The only question is, when?!?