Well, at the time although the war was very unpopular, I don't think a full pullout on short notice was considered a popular option even then. Even today I still am not sure that a majority of Americans would support just pulling the plug vietnam style.
that being said, if such a thing happened, you would probably be looking at Iraq turning into another middle eastern shithole within a few years, probably something akin to how Yemen looks today. The central government will probably sink further and further into corruption, and the iraqi military and police forces will be horribly deficient to deal with insurgency in the country (not entirely sure if they are even up to the task today). The Kurds will probably start a seperatist movement once it becomes clear that Bagdad will be unable to handle the job of running the country even on the basic level. If things get bad enough you may even see some of Iraq's neighbors launching military actions into the country to deal with instability, though that would probably require a really bad worst case scenario to develope.