I've always assumed that the norm against assassination, causally speaking, exists mostly due to historical promotion by leaders who wanted to maintain a low-assassination equilibrium, now maintained largely by inertia. (Of course, it could be normatively supported by other considerations.)

It makes sense to me that people would oversimplify the effect of assassination in basically the way you describe, overestimating the indispensability of leaders. I know I've seen a study on the effects of assassination on terrorist groups, but can't find a link or remember the conclusions.