[jfengel]

The United States never should have involved itself in Afghanistan.

That's debatable. Unlike Iraq, where the case for the invasion was dubious at best, the Afghan government really did host preparations for an attack on the US, and refused to turn over those who planned it. There's good reason to think that there would have been more attacks like 9/11.

That doesn't in itself justify war. Perhaps there were other means to prevent it. But war was not an unreasonable response, and it was largely supported by the rest of the world at the time. (Unlike the case in Iraq, which was widely opposed.)

As you say, having done so, we seem to have incurred a never-ending obligation to the residents for destroying their government. We didn't entirely wipe out the Taliban, and it will return to threaten both them and us. It's fairly clear by this point that we can't.

What we should do at this point isn't at all clear. The world is full of terrible things and we don't involve ourselves in all of them. Two decades of intense support could reasonably be said to discharge ourselves of the burden we picked up by invading. We suffer a moral loss, but having protected ourselves from more physical loss may be the best we can do under the circumstances.