It's also hard to see how "cleaning the slate" of a country's indigenous culture and institutions (as creepy as some of them are) jibes with respecting the self-determination of that very people.
This is the classic problem of how you enforce tolerance on the intolerant. The difference is sovereignty and representation -- we have the right to impose modernity on Mississippi, but not Kandahar, because Mississippi votes in our elections.
Going into Afghanistan to kill as many of AQ as we could was justifiable as national defense -- the Afghan government was sheltering and encouraging a direct, active threat to US lives that had just attacked us, and they refused to do anything about it. After the initial scattering, it becomes less and less justifiable and more and more impractical -- but it shades into it, there's no bright line. At this point, 8 years later, being in Afghanistan has become really, really expensive in a time of finite resources. The same argument for staying in Afghanistan until "the job is done" would also dictate that we immediately invade Somalia, Yemen, and probably a half dozen other countries where AQ is alive and well and plotting against us.
Just as with Iraq, it's time to leave, and if we leave we should leave entirely because any troops we leave behind will be vulnerable. If the people who really understand the situation (and it's unclear who they are -- the generals speak in good faith, but they have an enormous sunk cost loyalty to the current mission) think staying another 2 years will have a significant, long-term stabilizing influence, OK. If I had a vote, I would say we should remain at full strength until the day we leave, and then everybody out of the pool, because that would provide the greatest cover for our troops, but that would also create an enormous discontinuity for the Afghans which a gradual draw-down mitigates. Since we'd like to remain their allies, that's something to consider.
As usual, this is all way more complex than some opportunist's sound bite or bumper sticker.