I haven’t finished it yet but I found this fascinating. It is a study on terrorist groups from 1968 until now. It gives data on how often terrorist groups were successfully (not very often) and why they were successful (goals were somewhat narrow). What brought about their downfall (police, and negoitations with the government) and what indicators there are for a groups longevity (religion is strongly associated with longevity) and chances of success (religion is negatively associated with success).
Al-Qaeda doesn’t have a good future if history is any indicator.