I think that if US does not finish what it started and does not defeat Taliban and Al Qaeda as a fighting force in Afghanistan and Pakistan then either one of those countries (or both) have a high risk of falling into their hands. Then when the second attack on US is launched from there US will be forced to come back and start dealing with the problem once again only from scratch.
US certainly stirred the hornet's nest in the region by first invading and then neglecting it. Made Taliban from struggling governing entity into a powerful fighting force in the region capable of regime change. If US walks away now it will do so at a giant gamble to its own future security.
Don't cry, don't regret and don't blame
Weak find the whip, willing find freedom
Towards the sun, carry your name
In warm hands you are given
Ask the wind for the way
Uncertainty's gone, your path will unravel
Accept all as it is and do not blame
God or the Devil
~Born to Live - Mavrik~