It's not just that.
If we hand Afghanistan back to Al Quaeda and the Taliban ( which is what we will be effectively doing), we are going to get another attack on this country that will make 9/11 look like child's play.
What can we do is we withdraw and they attack us again? We'd have already effectively told them that we are not going to do much about it by withdrawing.
Your right. We have to kill every Taliban and Al Qaeda, because if we don’t, all they will do is regroup and attack us again. They are constantly thinking of new ways to kill us, we have to kill them. They probably already have biological weapons, nothing they have would surprise me