The problem, IMO, is a lack of coherent and achievable strategic goals in Afghanistan borne of a fundamental misunderstanding of the region.

Let's assume for a minute the Afghan government becomes relatively enduring and stable and that a large, capable and mostly self-sufficient National Army is created. At that point, Afghanistan will be the rough equivalent of where Pakistan is today. And just like Pakistan, it will be incapable of controlling large swaths of its own territory. And getting to that point (stable government, national Army) is a long way off if it happens at all.