What is the probability that the US will be able to train Iraqis to fight on their own? An e-history of the collapse of the South Vietnamese government in 1975 blames a number of purely military problems, but also mentions:
Because Thieu could not effectively eliminate corruption within his regime, the enticing, ideological arguments offered by the North Vietnamese were able to drive a divisive wedge between the people of South Vietnam and their government. In summary, I intend to show that the collapse of the RVNAF had not occurred suddenly in 1975, nor was the collapse due to any one single factor. Instead, the reasons were many: low morale, uncontrolled corruption, incompetent leadership, and the cutoff of U.S. military aid and air support. However, unless the government of South Vietnam could solve its own political problems, it was condemned to, not only losing the support of its own people, but the support of the American public and Congress as well. The Vietnam War was for the South Vietnamese to win. However, the U.S. could only provide aid to buy time for the government of South Vietnam to make significant reforms and rally the support of its own people to win the war against North Vietnam.
One can argue that the current government of Iraq has no need to "change the political order" as it's supposed to be an improvement on the political order in the first place. The fact that, yes there are many civilian targets of the insurgents in Iraq, but the US troops occupying the country are by far the main target for them, means that the current government of Iraq has a long, long way to go to be self-sustaining. As the insurgents are already self-sustaining in the absence of any clearly visible outside support and in the absence of a unifying political figure, the US-supported government of Iraq has to play catch-up. Is there any serious reason to believe that it wil last any longer than Thieu's government did after the North Vietnamese offensive of 1975 began?
It doesn't seem as though a 12-year struggle is in the cards, with or without American troops occupying Iraq for that whole period. America's support for the war is rapidly drying up and it doesn't appear likely that a self-sustaining government can be created under any circumstances, with any amount of material support.