However, the problem in this case is that this in an adversarial environment, and so publishing your decision criteria gives an advantage to the adversary. To make matters worse, we don't have a similar kind of insight into our adversary's strategic position, so it's very hard to determine whether the US is making progress. All we have is the observables. This differs from a conventional war where you have reasonable intelligence about the number of remaining enemy forces.
Imagine that you create some simple benchmark, e.g., the number of bombings per day has to be below X by December 2006 or we declare defeat. Now, come December next year, this goal hasn't been met. This could either mean that the adversary has lots of remaining capabilities or that they've concentrated all their efforts to meet this goal and force us out. (You'll recall the arguments that the Tet Offensive really represented a major depletion of North Vietnam's capabilities if the US had just been willing to stay the course). The above analysis assumes that the adversary's objective is to drive the US out. If they want to keep the US in and continue to bleed our resources, it's even easier to control the situation. Just back off a bit and let the US meet whatever milestones they have set, then start up again.
In general, it's an incredibly disadvantage to have your enemy know your strategic position when you don't know theirs. This is always a problem in democracies and the standard solution is to have a detailed strategy known to the leaders but only tell the public the very broad details. The problem, of course, is that this relies on the public being willing to trust that the leadership knows what it's doing, and now that the Bush administration has self-evidently lost that trust, it badly compromises the US's position.
You really should ask the question differently, "Ok, so you do want to withdraw from Iraq, now what?"