Thanks to the Project on Defense Alternative's compilation of exit plans for Iraq, I've just now been reading Steven Simon's booklet "After the Surge: The Case for U.S. Military Disengagement from Iraq" for the Council on Foreign Relations that contains what is, I think, one of the best diagnoses of the problem:
Leaving U.S. forces in Iraq under today’s circumstances means the United States is culpable but not capable—that is, Washington bears substantial responsibility for developments within Iraq without the ability to shape those
developments in a positive direction. In consequence, Iraqi support for the U.S. presence has collapsed. Polls indicate that most Iraqis want the United States to pull out. Moreover, the Iraq war has fueled the jihad and apparently been a godsend to jihadi recruiters—and the process of self-recruitment—as indicated by the 2006 National Intelligence Estimate (NIE) on the global war on terror. More broadly, the Iraq war has had a very damaging effect on the U.S. reputation in the Arab and wider Islamic world. Authoritative opinion surveys show this as well. The continued presence of U.S. forces is thus a severe setback in the canonical war of ideas, which the Bush administration has correctly assessed as crucial to American interests. [...]
In 2004, Ayman al-Zawahiri, the deputy to Osama bin Laden, said of the U.S. intervention: “America is between two fires. If it stays in Iraq, it will bleed to death; if it leaves, it will lose everything.” His forecast comes disturbingly close to describing current circumstances. It need not, however, be prophecy. More than three years after the intervention began, to be sure, the United States finds itself in an agonizing strategic position. The time has come to acknowledge that the United States must fundamentally recast its commitment to Iraq. It must do so without any illusions that there are unexplored or magic fixes, whether diplomatic or military. Some disasters are irretrievable. Having staked its prestige on the intervention and failed to achieve many of its objectives, the United States will certainly pay a price for military disengagement from Iraq. But if the United States manages its departure from Iraq carefully, it will not have lost everything. Rather, the United States will have preserved the opportunity to recover vital assets that its campaign in Iraq has imperiled: diplomatic initiative, global reputation, and the well-being and political utility of its ground forces. [...]
But raising the prospect of desperate deterioration in Iraq and its environs after an American military disengagement necessarily tends to obscure two things. First, the presence of U.S. forces has not stabilized Iraq thus far. Second, conditions for instability have become structural elements of Iraqi politics. Given these facts, how long should the U.S. keep troops in Iraq, when its military presence only delays an inevitable escalation of intra-Iraqi fighting?
I still hear it often said, including by liberal-minded people, that all serious experts agree that we need to stay in Iraq, or even that the consensus on this score is so overwhelming that it's inevitable that we'll stay. Neither is true. Quite a lot of who've thought deeply about this problem have concluded that the best thing to do is simply to cut our losses and leave, focusing our remaining Iraq-related energies on doing what we can for refugees and to improve the broader regional diplomatic situation.