You obviously you missed the point. You are just so obtuse. I guess I have to spell it out to you. Imagine that the Nazis were still around but underground 15-20 years after being defeated. However, once the US pulled out of Germany, they had the power to sweep in and take the country over by force. That is the analogy.
So having troops in Afghanistan would not be beneficial to the US interests since it borders on hotspots like Iran and Pakistan? That is not strategic? I don't know what your definition of existential threats to America, but besides Japan and England, there has never been a foreign nation that has attacked the US within its borders. However, this country has been attacked consistently by islamist terrorists since 9/11. And, if you think the mission was complete a few months after 9/11, you have no idea what you are talking about.
The mission is never complete. The war against terror remains. Was it just three years ago when a van ran over people in NYC, Boston Bombers, Pensacola shooting of naval officers, what about the the Chelsea and Seaside Heights bomber? The list goes on and on.
Iran is trying to be a nuclear power and Pakistan's (already a nuclear power) more extreme islamist element has its brother back in the Taliban in Afghanistan. The Taliban is a terrorist group that will regain control of Afghanistan once again. That should scare you. Perhaps you should go and visit the 9/11 museum.It might do you some good.