Al Qaeda is not an organization that commands massive resources. It doesn’t have a big army. It doesn’t have vast reservoirs of funds that it can direct easily across the world.
Al Qaeda was an idea and an ideology, symbolized by an extremely charismatic figure in Osama bin Laden. Bin Laden was this Saudi prince-like figure who had gone into the mountains of Afghanistan forsaking the riches of a multibillion-dollar fortune, fought against the Soviets, demonstrated personal bravery and then crafted a seductive message about Islam and Islamic extremism as a path to destroy the corrupt regimes of the Middle East.
History teaches us that the loss of the charismatic leader - of the symbol - is extraordinarily damaging for the organization. It is very difficult to keep such an organization together, particularly in the absence of great power backers.
In the case of al Qaeda, this is a virtual organization held together by its message and the inspiration it provided. A large part of that inspiration was bin Laden. Ayman Zawahiri may have been the brains behind the outfit, but he did not excite people. When people volunteered for jihad, they were volunteering to be bin Laden’s foot soldiers, not Ayman Zawahiri’s or Khalid Sheikh Mohammed’s. The loss of bin Laden’s personality is hugely important because it was so much part of al Qaeda’s appeal.