Foreign incursions aren't that likely, contra Cheney-94. The Shia won't rush to join Iran and the Sunni won't rush to join Saudi Arabia or Syria (which is run by Alawites anyway).
To answer the question, the US government can not fix the problem in Iraq. Downshifting goals from "Create a utopia" to "fix the infrastructure" or "train enough troops" doesn't help. Keeping a third of the troops there to do nothing unless things get Even Worse, and then expecting this smaller force to deal with a bigger problem, won't help.
Whoever is president should go to the generals, say "I want the troops out of Iraq. How soon can we do that without putting them in danger or putting an unreasonable logistic strain on the military?"
If Biden wanted to get the troops out ASAP, but was warning people that the process could take months, that's one thing. If he's pushing the idea that the US needs to stay until certain goals are accomplished, even if those goals are more modest than Bush's, he's wrong.