Money is the big issue. There is oil in the Kurdish North, Oil in the Shiite South, and bupkis in the middle where the largest population the Sunni live. They are not going to accept it if the oil revenue isn't divied up. It's reality.
The Shiia are the largest population, they are 60% while the Sunni are only 20%
They have nothing to bargain with. The current government benefitted the Sunnis more than the Kurds or Shia. They've been sabotaging it. They've been pi$$ing in the well, and now they don't like the way the coffee tastes.
Am I going to care if the Sunnis start a war with the Shiites or the Kurds, and they get their @sses thoroughly kicked?
Not likely...not in this lifetime anyway.
What's not to accept? The Sunnis will be poor, and badly outnumbered. 80% of the Iraqi population will have plenty of oil money coming in, and won't think twice about killing Sunnis who cause trouble. Once we're out of the way, and the Iraqis can settle their own mess, the Sunnis will wish they had played ball earlier. When it mattered whether or not they 'accepted' what was going on, that is.