I have never been persuaded of such devices, but RAND does keep on producing:http://www.rand.org/pubs/research_reports/RR1273.html

The two key points for me:
Two factors remained absent in Afghanistan in 2015 but essential to success in historical COIN campaigns: disrupting flows of tangible support to the insurgents and a demonstration (and improvement) of commitment and motivation on the part of the Afghan National Security Forces, the primary COIN force since the coalition drawdown.
Three of the recommendations are laughable and have probably been said so many times before.

Would RAND or another other contractor "think tank" say "Enough, we've been there long enough, time to go"?