What would insurgents do if the U.S. quit fighting in Iraq?

This is from Mark Kleiman.

"There is, in fact, no Iraqi insurgency. There is a Sunni Arab insurgency. And it cannot win."

This deserves as wide circulation as possible. It prompts several thoughts.

The Sunni insurgency has two components: secular Sunni ex-Baathists whose programme is more or less (a) "Bring back Saddam (or another Sunni Arab secular strongman), lording it over Kurds and Shia" and Sunni jihadis who want (b) "drive the infidel Americans out", and (unclearly) (c) "set up a Sunni theocracy. lording it over Kurds and Shia".

Thought experiment: if the USA just quit tomorrow, what would the insurgents do? The jihadis would have achieved aim (b); since aim (c) is suicidally impossible, they would most likely declare victory and move on. That would leave the secular Baathists. The Kurds would stand on the sidelines while the Shia militias crushed them with Iranian help. Ethnic cleansing of defeated Sunnis would be a possibility. End-state: de facto partition of Iraq into two (think Belgium or Bosnia), with an ongoing low-level Sunni terrorist movement (think ETA, IRA) preventing economic recovery in the Arab part but not strong enough to change the regime.

Is there really any reason for us to remain in Iraq?

If we pulled out tomorrow, would the Sunni insurgents be able to put together a conventional force that could retake control of Shia and Kurdish Iraq?

Of course, the U.S. would not have bases in Iraq, nor would there be any control over Iraqi oil. The grand demonstration project so beloved of the NeoCons in the heart of the Middle East would have to go forward without guidance from Americans.

The Iranians would be providing military support to the Iraqi Shiites, but they have already agreed on that. The Kurds, disliked by both the Iraqi Sunnis and the Iranian government would be up a creek, but the Kurds are used to being used and abandoned. We've done that to them before when the Iraqi government under Saddam attacked them.

Haliburton stock would take a big hit.

Some America GI would be the last American killed in a lost war, but that is going to happen anyway. The only questions are who, and how many others will there be between now and then.

Much of the motivation of the population of Iraq to support the insurgents would disappear.

Where is the downside of us just announcing our evacuation tomorrow?