They already had a civil war. Saddam quelled a big uprising among Shi'a and Kurds. Unity among the Muslim factions is stronger than national unity, so your proposed consequence already existed for 1000 years.
Yeah difference being those conflicts didn’t involve raining 1000s of bombs down over major cities. The situation for civil war already existed, so the best option is to light the fuse by bombing the place and leaving a power vacuum? Not a great plan to eliminate terrorism or promote stability. It’s not hard to think of better ways to spend tax dollars and avoid the emotional toll of losing many good people.
