1. Baghdad is not going to fall. No way. Those other cities fell quickly due to a corrupt army, and non-Shia populations hardly sad to see it go. The price for Obama letting Baghdad fall is abject US humiliation, as Richard Fernandez spells out. Far more importantly, the price for non-Sunni and non-Islamist Iraqis letting it fall is slavery and death. Perhaps ISIL will make a few dramatic raids and more bombings, but that’s it.
2. The ISIL strategy is to provoke Sunni-Shia civil war, and making it so that they are the main organization remaining for Sunnis to rally around. But there are plenty of Sunnis who had already been pushed to the brink of open rebellion and beyond by their mistreatment by the Maliki government. See the excellent article on this and whole situation by one Paul Mutter, at The Arabist.