It seems to me that it is entirely possible that neither side can win: that the international community cannot defeat the insurgency or build a stable, functional, Afghan government that can assure security and exercise effective control over large parts of the country, while the Taliban cannot (given both their ethnic and sectarian opponents, as well as rivals within the Pashtun community) capture Kabul as they did in September 1996.

I think I've made this point before, the real risk—from an Afghan perspective---is that this become the prolonged reality. The international community slowly disengages from a COIN fight is can't win, but throws enough money and guns at the ANA/ANP, the ex-Northern Alliance, southern warlords, and others (including Iranian support to Hazara militias) to stalemate the Taliban. The Taliban, on the other hand, consolidate practical control over parts of the country, while fighting a continued civil war.

The depressing model here is Lebanon, 1975-90. Everyone (Israel, Syria, Egypt, Iraq, Saudi Arabia, the PLO, Iran, etc.) simply threw resources at local clients in such a way to prevent their opponents from 'winning."