So? You keep killing the terrorists until there are no more. That goes for their admirers too.
“They (the Romans) make a desert, and they call it peace”
The trouble is that your course would lead to an expansion of terror and imitators across vast swathes of previously unaffected nations and matches Al Qaeda’s dreams exactly. Witness their delighted attempts to make the tradition Islamic anger points resonate in nations like Indonesia and others traditionally disinterested in the more Middle eastern issues.
The more martyrs we create the stronger Al Qaeda (or imitators) become. Yes, all might be destroyed in the end but it is by no means certain this can be achieved without severe damage and cost which might be avoidable by clever tactics. Sometimes killing is the only way; at other times the same end can be achieved without turning the target into a rallying cry for a wave of new recruits.