I'm pretty sure the Talibans (not to defend them, mind you), were already cracking down on poppy farming before 9/11 and the subsequent decade long war.

So how were they benefitting? Or do you mean to say the US and allied forces allowed mass poppy crop farming that was then utilized by the Taliban to fund itself? You know there is an alternative hypothesis: the US and other occupation allied forces tolerated poppy farming to pacify and win over tribal chiefs and keep corrupt Afghan officials squarely on their side. Maybe both were happening, who is to say.