I think Zawahri's toast without Bin Laden. He's got a serious problem with the "hey you kids, get off my lawn" thing:

Both of 'em were bascially has beens among young jihadis, but Osama really truly did have a charismatic religious talent that resonated with the ultra devout (I remember seeing a video of a young Saudi woman saying stuff about his videos like he was a rock star and she was a groupie) and he would still have maintained a certain "saintly elder" rep among some.  As one relative said about him as a young man, he was "holy holy."

Zawahri on the other hand, was always the angry guy, he's always just bitchin' or crabbin' about someone or something or another, logistics is all and there is no inspirational quality there. . None. It's all political Islam, no holy Islam, all rules, no dreams, even looks like personal grudge sometimes. If there's any young true Al Qaeda believers left, and even that's questionable (especially with "Arab spring" movements allover the place.) Zawahri is not the one they'd follow,  Even Mullah Omar would be a better candidate-he's got the holy uneducated hermit thing. Al-Awlaki has his fans but he tends more to the Zawahri angry rather than the Osama inspirational.