I had the pleasure of being here when Barnett Rubin came a few weeks back. He wrote a book called The Fragmentation of Afghanistan long before anybody was interested in Afghanistan.
One of the key findings that he put forth in his book, looking at the last two centuries, was that Afghanistan has only been able to sustain a political apparatus when it has had outside funding sources, whether through occupying part of India, having a deal with the British, being sustained by the Soviets, or having Iranians playing off one donor against the other.
The question you were asking is a very important question. Lakhdar Brahimi, who was the former head of the UN mission that was there in the 1990s, mentioned this to me in a private conversation, saying that his biggest worry was actually sustainability of what we were trying to accomplish in Afghanistan.