Tom Ricks' position on a residual force, specifically that although he had thought it would have been a good idea at the time, if possible, in retrospect it would have been a costly mistake -- with the present sectarian warfare, U.S. forces would have required significant reinforcement, or would have had to withdraw from combat, with either of those outcomes being worse for the U.S. than leaving the conflict to the incompetent hands of the Iraqi army.
At the time the Obama Administration was attempting to negotiate a new status of forces agreement, the American public didn't want to keep combat troops in Iraq, the Iraqi public didn't want our forces there, and the Maliki government also wanted the U.S. out. The argument that President Obama could have used his powers of persuasion to maintain a significant presence of U.S. troops, despite the opposition of everybody affected by the decision, is an interesting one, but to me it exists only in the realm of fantasy. There's nothing surprising about ethnic tensions in Iraq, nor that the actions of the Maliki government have worsened ethnic tensions.
It's reasonable to infer that if Maliki foresaw this type of breakdown and renewed civil war, to the extent that a U.S. military presence would have prevented the problem he would have wanted U.S. forces to remain. Given his administration's secular favoritism, cronyism, and incompetence in its management of the Iraqi army, it's reasonable to infer that one looming factor in his wanting U.S. combat forces out was that they might delay or prevent the implementation of his plans -- plans to reward his cronies and advance a sectarian form of government. Nothing about Maliki should have been a big surprise by the time U.S. combat forces left Iraq, so where's the evidence that Maliki would have changed his mind, upsetting the Iraqi people and alienating his friends in Iran, had Obama said "pretty please", perhaps "with sugar on top"?
Contrary to the apparent beliefs of the fantasists, I don't believe that the U.S. presence would have delayed civil war forever, or even for more than a couple of years (if that). Why not? Because Iraq experienced a civil war during active U.S. military occupation, with essentially the same parties fighting it out. The war, occupation and "surge" put a band-aid on the civil conflict, covering a festering wound. It would have taken true commitment to the cause for a central government to even partially heal that wound and, if we're honest, the Bush Administration knew from the earliest days of Maliki's governance that the Maliki was not the man for that job.
For some of the advocates of a residual force, such as John McCain, the position seems rooted in traditional wars between nation states. The U.S. is involved in a war and, when the war ends, it leaves a significant military presence in the nation where operations occurred in order to secure that nation from a potentially or overtly hostile neighbor. McCain apparently sees no distinction between wars between nation states and civil wars, despite his own experiences in Vietnam both during and after the war. Suffice to say, there's an enormous difference between maintaining a military force that is supposed to keep a lid on civil unrest and ethnic tensions, and one that sits near a border to intimidate a neighboring nation out of trying to cross that line.
Other than dragging the U.S. back into an Iraqi civil war, something few other than perhaps John McCain would actually favor, what would have been the benefit of a residual force? The best case scenario seems to be that the U.S. presence would have delayed the inevitable, but the worst case scenarios that Ricks describes seem far more likely.