We haven't killed everyone yet
I read this and I immediately thought I should just let it slide, since I figured that, seeing as how you made this your justification, you must be fifteen or sixteen years old; curiosity got the best of me and I checked your profile: says you are thirty-four.
Now, this could be wrong. The birth date I have on here is the anniversary of the Battle of Corregidor; my way of honoring my grandpa and his brother. So, you may still be fifteen or sixteen.
Unfortunately, unless you state otherwise, I am going to give credence to your listed birth date. That being the case, I look at your post and I am drawn to one of two conclusions:
1. You literally mean that we have not killed everyone residing in Afghanistan and until we commit that genocide, we should keep our troops there.
2. We have not killed all of the terrorists operating within Afghanistan, and we should keep our troops there until we have done so.
I am going to give you the benefit of the doubt and assume you mean to kill the terrorists. In your opinion, what is the highest cost (in terms of money spent and American lives lost) you are willing to pay to root out every last terrorist in Afghanistan?
Do you believe there are more efficient ways to interdict and disable terrorist cells than through the use of conventional troops?
Do you believe that conventional troops do a good job in counterinsurgency situations?
Do you believe that Karzai genuinely seeks to advance an agenda that conforms to US interests?
Do you believe that any possible replacement for Karzai would advance an agenda that conforms to US interests?