Because the number of troops that can be reasonably demanded for a COIN operation is essentially limitless, mission failure can be blamed on the executive branch for not sending enough troops rather than on military leaders, the combat environment, or the COIN playbook itself. As Gen. McChrystal wrote in his assessment: "Success is not ensured by additional forces alone, but continued underresourcing will likely cause failure."
Perhaps in anticipation of eventual accusations of underresourcing National Security Adviser James L. Jones in a SPIEGEL interview states:
I believe we will not solve the problem with troops alone. The minimum number is important, of course. But there is no maximum number, however. And what's really important in Afghanistan is that with this new administration we insist on good governance, that it be coordinated with economic development and security, and that we have much, much better success at handing over responsibility for these three things to the Afghans.This apparently prepares to blame an eventual defeat in Afghanistan on the Afghans.
It is a safe bet that "who lost Afghanistan" will be argued inconclusively for many years afterward. Perhaps 22nd century historians will arrive at a "settled" and generally accepted version -- and that, I predict, will be that the venture had so many causes of failure recognizable in advance that the only hard to explain aspect is why it was attempted in the first place. (You just had to be there).