Good question. Why is it realistic?
Afghanistan is larger than Iraq, has a larger and equally aggressive population, a much more forbidding geography, a worse climate, a major al Qaeda/Taliban enclave right across the border, and even more tribalism and internal violence than Iraq.
Yet somehow it has always been assumed by the anti-Iraq crowd that, given the same number of troops, and the elimination of a single al Qaeda leader, that success would have been assured in creating a stable nation.
You tell me.