It's inconceivable that an Al Qaeda linked orgnisation could take over power in Iraq following an American pull-out. The only thing that has given them any possibility of operating has been the American presence.

Of course it's also pretty inconceivable that there will be an Iraqi regime that is particularly friendly to the USA either, but that is another matter. But staying longer isn't going to do anything to change that, except perhaps by making things even worse in that respect than they would be otherwise.