Wouldn't Al Qaeda simply roll back in and attack us again? Wouldn't we look impotent on the world stage?
No and No. After a withdrawal...and if....Al Qaeda tried to re-establish themselves in Afghanistan, we have a myriad of options at our disposal that are far more lethal and far more conservative than perpetual occupation.
Remember....we're not even fighting Al Qaeda in Afghanistan....we're fighting the 'Taliban' [entities]...who didn't attack us, don't have global jihad aspirations, and simply want us to leave. Why again are our sons and daughters fighting and dying there?
To start, we now have an incredibly detailed picture of the human and geographical terrain in Afghanistan, compared to September 2001. With that, we have HUMINT sources, SIGINT signatures and IMINT data to detail any occurrence of a 'training camp' being established post-withdrawal. Every arm of our military can contribute to a post-withdrawal reaction force, should Al Qaeda attempt to establish itself again [though nobody has anything beyond hypothesis that this would even occur, as AQ has the same information that I just posted above]. Having Al Qaeda back in Afghanistan would actually be to our great benefit, as they could then be interdicted. That's the myth of a 'safe haven'. Terrorism 101: establish a static position, and you are vulnerable to attack.
Predators already fly out of bases near Islamabad and in Uzbekistan. Army Special Forces would continue to aid the Karzai regime, or in the event of a coup, tribes that would be anti-Taliban [as we did with the Northern Alliance with great effect]. SF teams, Navy SEALS, MARSOC and three Ranger Battalions are all trained to strike and interdict in a non-permissive environment from Carrier Task Groups or friendly airbases. And Special Mission Units (SMU's) are an even more highly trained and specially focused element to conduct HVT grabs, raids and ambushes. Airspace permissiveness is not really an issue beyond Stinger and other manportable air defense systems that the Taliban already possesses.
The savings in terms of dollars, manpower and most importantly lives, would be tremendous for obvious reasons, but also the not so obvious, such as force Protection, which eats up an enormous chunk of the war budget and manpower.
If Al Qaeda returns and holds ground in Afghanistan, we have every opportunity to strike at them. Host government authorization doesn't appear to be an impediment, as we are striking with impunity inside Pakistan, while the government outwardly condemns them.