Interesting point. However, I am not sure overwhelming numbers would have ultimately meant a United Iraq. But you are correct that the moment we left things went south. The day we left Maliki (Shia) arrested the VP (Sunni) with no US reaction. So is Obama’s policy to have a Pan-Kurdish and Pan Sunni state in what was Iraq and Syria? To be noted that Joe Bidden advocated this position long ago in the Senate (before 2006).
I’m not so sure Iraq in the best of circumstances can be a united country. Too much bad blood between the Sunnis and Shiites with the Kurds stuck in the middle. The Baath regime was only able to hold things together through brutal means.