I think it would be better to do it Alexander's way, who left a garrison but as a rally-point at which he could suddenly appear with his main force and start scragging the backstabbers (Afghans have always been championship weasels and backstabbers).
Don't leave a huge force to be an irritant, but the one that remains has to be able to defend itself until the cavalry gets there; and they should be taken out, too, when the USG is satisfied that the Afghan government of the day is not going to ally with the Wahhabist Pathans again and invite Al Q'aeda back in. Denial of Al Q'aeda is the key.
That is an interesting alternative to complete withdrawal, which, sooner or later, would be preferable. Having some measure of security for the remaining forces might entail a significant continuing commitment, and I am not sure how that would play out politically unless the national government were friendly and stable. We still maintain considerable presence in places like South Korea, but as long as there are no continuing hostilities to generate casualties and thus domestic discontent, the public seems to accept it.