Leaving the Iraqi old guard in charge and having Saddam still out there would have been an even bigger disaster. I don't buy the thinking that you go to war with a country and than turn it over to the previous military within months.
I didn't say you would keep Saddam propped up. Or keep the old Army in charge. The old Army would be supervised and under control, not freelancing with an insurgency. The Occupation would still take years.