The largest democracy in the world is India, just a little gift from the British. And pluralism is a more prominent feature of democratic societies than "unitary." Most people in Iraq aren't Baathist or religious fanatics.
Just because there are many difficulties ahead doesn't mean it is impossible. Quite to the contrary.
India had a long history as a nation and the Indians have a centuries long sense of nationhood. Iraq (which was thrown together arbitrarily like Bosnia or the Congo) has no such history.
India was able to avoid genocidal civil war because the Muslim areas were turned into Pakistan. As a result, the Hindus are in the vast majority and it is, more or less, religiously/ethnically homogeneous. By contrast, the U.S. has vowed *not* to allow this sensible partition option to be applied Iraq. Instead, it has adopted a policy of *forcing* the Kurds, Sunnis, and Shi'ites to stay in a unitary state.
Imagine if the British had tried to keep India together in 1947 (rather than allow the partition) and you get some sense of what the future holds in Iraq.