Correct. Southern Iraq is Shiite, and they have their own militias. A rag-tag band of sunni jihadis wouldn't dare enter Baghdad. ISIS may create chaos and trigger all kinds of violence and retribution, but they won't enter it, much less control Baghdad. My guess is Maliki felt the sunni towns of Mosul and Tikrit were indefensible anyway and his shiite-led government probably wasn't popular there anyway - so he let them go. As for the North - The Kurds have had effective independence since 1991 - there is not a chance they lose it to a bunch of sunni crazies.
I bank on the Sunni—They have the drive—they have expeience on their side—If they have a good general they will flank the defenders—cut them off and make for Baghdad itself. If it falls they will have the state. Iran may have to send more than a few “Revolutionary Guards” Send in their air force and heavy tanks. After Baghdada falls they will head for the Kurds in the north. Next stop the Oil Rich states on the Gulf. Then Israel and the USA—Lots of folks going to die before the year is out.