bin Laden's cadres aren't the Afghan people - they're fanatical ethnic Arabs and other fanatical people who aren't from Afghanistan.
While helping the Afghan people is something we certainly need to do, it's not going to stop the terrorists we know are out there. They'll have to move out of Afghanistan if the US is successful, but many/most of them are scattered around the world already ("cells in 60 countries").
I don't think it's a solution to the immediate problem.
Cheers,
Scott.