Will The Occupation Effectively End Dec 31?

The legality of the Iraq occupation unravels without a SOFA. But the SOFA may not get past the parliament and a UN resolution won't get past Russia:

U.S. officials do not dispute that the absence of an agreement would probably require an immediate end to combat operations and, at a minimum, confinement to bases on Jan. 1. Officials refused to discuss the sensitive issue on the record while negotiations are ongoing.

Why not let it expire and withdraw all troops swiftly thereafter? If the Iraqis don't want it and the UN won't approve it, why should the US insist on spending money and risking lives in an occupation that is already more than five years old? If the US is alone in wanting to stay in Iraq, it really does become an empire, no?