It is inevitable that at some point the USA (and the "coalition") will pull out and leave Iraq to its own devices, just as it was inevitable that it would pull out of Vietnam. It was predictable and predicted that it would end that way even before the invasion happened - just as it was predictable and predicted that the outcome of the invasion would be disastrous for Iraq.

After the USA pulls out, whenever that is, it is likely that things will be very bad indeed. But staying doesn't make that any less likely, it probably makes the outcome even worse.

The most likely end game (and the most hopeful of a very bad bunch of alternatives) would be be some kind of partition, possibly informal, with Iran having a sphere of influence in the Shi'ite South East, and Syria in the West, and the Kurdish North continuing to be effectively autonomous, by not officially independent. Maybe the Iranians and the Syrians might have a better chance of reducing the sectarian killings, and negotiating some kind of stand-off in the mixed areas.

The end result one way and another is probably going to be worse than it was before the invasion. Even for those who aren't among the hundreds of thousands dead.