It's at least worth us taking a shot at doing what we can to help the creation of a stable coalition government there, if possible.
Otherwise, the Taliban or some other radical group could well be in charge in twelve months, and we'd be in the same situation we were on 9/11, all over again.
Maybe that's inevitable anyway, but we can't be sure unless we give it a try, and thus we should indeed try.
It's like the old Western movies -- we're kicking out the Taliban because they're like the corrupt sheriff who was actually part of the criminal gang tearing up the town (and the surrounding towns). But just kicking out the corrupt sheriff accomplishes little if you don't make sure to fill the vacuum with an honest sheriff who will nip any future crime in the bud. Otherwise any remaining thugs can just take over the town again as soon as you ride out into the sunset.
It's in our own national interest to try to help the good guys in Afghanistan (or lacking that, the less rabidly anti-American guys) to take and hold power in Afghanistan.
Personally, I'd rather not have to invade Afghanistan every four years to track down the latest band of government-sponsored terrorists. The best way to do that is to try to build the foundations of an Afghani government that will prosecute terrorists instead of support them. The best way to do that is to give the new government more incentive to do business with us than to attack us. But that won't happen if we just kill off their old government then "get the hell out", as you say.
We need to stay, at least in spirit, and build a relationship with the new government there. Or else we'll be back to square one in a few years.
Maybe that's inevitable anyway, but we can't be sure unless we give it a try, and thus we should indeed try.
We've "given it a try" too many times to count, from Somalia to Bosnia to Haiti to Kosovo and so on and so on. There simply is no record of a single success; just a ton of wasted taxpayer money, enriched corrupt officials and Western consultants, an impoverished population, and authoritarianism. What is the saying about continuing to do the same thing and expecting different results being the definition of insanity?