There is no winning move in Afghanistan: the President who pulls out will be blamed when Afghanistan returns to its pre-war state, and political enemies will chalk it up as a "loss."

There might be a chance of ending the war during a lame duck presidency, but that President will be thinking of Legacy at that point.

Thus the only politically-viable course of action is to keep the war going. As a sibling comment mentioned: war serves more than one purpose.

Unless of course the culture of America shifts to an anti-war or non-interventionist stance and sees peace as more important than "winning" or "losing" in Afghanistan. Unfortunately, I don't see that happening any time soon.