Would Hannibal believe the US war in Afghanistan is winnable?

Likely, yes.

There's a caveat here, however.

One who has superior military power can win the war if that is indeed the objective of that force.

Some evidence suggests that both the US policy as well as our unstated policy is not actually to "win the war" in a traditional sense, but to create a new power dynamic that institutes long term security for the region (and vicariously, the US and her allies).