From "Academics Doubt Impact of Osama bin Laden’s Death":

"...Fifty-three percent of the terrorist organizations that suffered such a violent leadership loss fell apart — which sounds impressive until you discover that 70 percent of groups who did not deal with an assassination no longer exist.

Further crunching of the numbers revealed that leadership decapitation becomes more counterproductive the older the group is. The difference in collapse rates (between groups that did and did not have a leader assassinated) is fairly small among organizations less than 20 years old but quite large for those more than 20 years in age, and even larger for those that have been around more than 30 years.

Assassination of a leader does seem to negatively impact smaller terrorist groups: The data shows organizations with fewer than 500 members are more likely to collapse if they suffer such a leadership loss. But organizations with more than 500 members are actually more likely to survive after an assassination, making this strategy “highly counterproductive for larger groups,” Jordan writes."

See also Lost Purposes, The Importance of Goodhart's Law, & Faster than Science.