I see where you are going with this. What if a future U.S. president decides to pull out, caving in to pressure from the spineless Republicans in Congress and we are left with, as you say in your clsoing paragraph, essential a terrorist state with all the money they can literally pump out of the ground. What would happen? What would we do?
Sadly, this is an all-too plausible a scenario. That is, Republicans going soft; calling for timetables for withdrawal, etc. (see Capital Advantage's letters to Rep. Jones http://www3.capwiz.com/y/bio/letterslist/?offset=0&length=10&issue=1&id=442).
And you don't need a crystal ball to know what will happen. Just turn to any reputable history of the Vietnam War and its aftermath. There will be refugees, mass murder, etc. You would think...hmmmm what can we do differently in Iraq to avoid the mistakes of Vietnam? Well, we could ruthless round up, mercilessy interrogate and summarily execute every mother son of the insurgency until "Baathist" becomes a word that is no longer uttered in polite company; it should become the most vile and ugly thing you could say about a person, much like the "N-word" is today.
We are never going to win the hearts and minds of those in the insurgency, so those concerned with our "image" as that of Gitmo' and Abu Grahib should consider this fact. There was no Gitmo, no Abu Grahib when civilian airliners were hijacked and used as missiles. What was our image problem then? Well I'll grant you, lobbing cruise missiles at an aspirin factory was a stupid mistake. So was committing troops to Somalia, and stupider still was pulling them out after Mogadishu.
So we should concentrate on winning the hearts and minds of those tens of millions of Iraqis who voted in January for a real government. The de-Nazification of Germany following the war was barbaric by today's standards, but it was successful. The allies and German civilian groups rounded up those remaining in Nazi cells hiding throughout Europe. Brutal but necessary to demonstrate to the civilian population that it was safe to go about life, run for public office, take financial risks by opening or improving a business, etc.
Anyway, in your closing paragraph you wonder about a murderous regime financed by oil. Well we have had them. One is gone, Saddam Hussein's murderous reign is over. We still have Iran, though. They have been financing suicide bombings in Israel for decades.