Even the Afghanis hate Karzai, they don’t trust him, they know he is corrupt. Massoud was making great headway until his assassination.
We leave and give in to the Taliban we are back to square one again. The country then goes up for grabs to any terrorist entity that will pay the price and they will have safe haven to do whatever they want. Plus do you really want a power base right next to a nuclear Pakistan? They are teetering as it is.
I know people aren’t supposed to draw parallels to Vietnam, but the fact of the matter is the Taliban have a lot of popular support (often more than the central government). They also don’t require much to field fighters, and have nothing but time on their hands (and on their side). What it would take to beat them nobody would ever allow our troops to do. The fact is that in much of the countryside the Taliban move freely, and the central government is only propped up by our troops; it has little authority, and has negotiated with the Taliban (even threatened to side with them) because it understands they have a role there.