I'm sure you recall Al Haig's "I'm in charge now" statement. Put that in play with a dozen ISIS leaders who aren't 100% positive who's right.
I think this is actually making my point for me. The point here isn't how well-developed the chain of command is, it's the size of the leadership structure. If there are upwards of a dozen guys with a legitimate claim - one which at least some adherents believe - to be the leader of the movement, that means you have to kill a dozen guys to leave them without a leader. May not speak all that well to their organization, but that again furthers my point. Clearly no one individual or influential group is centrally planning all of their activities. That means that killing a small group of leaders isn't going to stop all of said activities.
The elephant in the room that you haven't even tried to address is the presence of essentially a military at this point. They have at least 50,000 armed militants within the central territory, and possible 4-5 times that number. Do you think all of those guys are just going to fade away quietly if we kill a handful of leaders they may or may not even recognize as such?