The Taliban's strategy has openly been to wait for the NATO forces to leave and declare some phony half-hearted victory. Then they have sworn to take that squalid country back to its seventh century roots of a tribalistic, male-dominated Muslim society which imposes warped cultural values on the will of those least able to defend themselves.
That country is so unstable that some kind of internal conflict is going to unfold regardless of whether the United States stays or leaves. The country has many ethnic and tribal conflicts, and it is not a traditionally unified or well-organized area. They are going to fight no matter what we do.
How will we ever know if we don't do it? It seems absurd to maintain outside forces in a country because we aren't sure if that country could maintain stability on its own if we withdrew. I guess it is important to us that they do maintain stability, otherwise we wouldn't be there, but we need at some point to remove ourselves and allow them to give it a go. Otherwise, we may as well plan on staying there forever.
There already is a civil war in Afghanistan, between the American backed Karzai government and the Taliban. In fact, Washington withdrawing from Afghanistan may actually lead to a resolution to the conflict, as the Karzai government would probably be overthrown quite quickly. At least America's withdrawal will mean fewer massacres and drone strikes.