With all due respect, there is more to the story.
During the 2008 election, an extension of the above status of forces agreement was being negotiated.
And, while campaigning for the presidency, Barack Obama took it upon himself to make a trip to Iraq and meet with the Prime Minister. Nobody knows for sure what the content of their discussion was but, from that point forward, Iraq's negotiators for the status of forces agreement became totally uncooperative.
The assumption is that Obama promised he'd give Malaki what he wanted -- and couldn't get from Bush.
Consequently, I wouldn't be so quick to blame the lapse of the status of forces agreement and the early exit on Bush.
The fact of the matter is that Bush waited until after Obama had won the 2008 election to sign that agreement -- for whatever that's worth. If what you've said is true, then why would Bush negotiate an agreement at all?