I suspect that historians will show that it is about 50/50 that an "installed" government, even a loosely democratic one, will long endure, once the installer has moved on.
Therefore, since the British are already pulling out shortly, we should do the same, with the realization that we are likely going to have to re-engage, with a 50/50 likelihood, whether we wait for "assurances" that only 'more time' will be presumed to bring or press the issue(s), now.
We have no metrics for benign political sinews that may be strengthening, in Iraq. Accordingly, the military's progress indicators are probably a statistical fool's errand.
It might be best to stress the system, to see if it can stand on its own, rather than continuing to pay for training wheels and running alongside ... either that, or finally get someone else to start paying in full for continuation (like the neighboring states).