On the BBC or Channel4 news type programmes in the UK, a while back, investigating "islamic terror extremists", there were several academic types who appeared on the programme to mention how Usama Bin Laden spoke "very beautiful Arabic". They were very enamoured of how well he spoke.
So, Usama was a well-spoken, intelligent man. And the al-Quaeda organisation, even though Bin Laden wasn't directly commanding things, would take a lot of its morale from him. Eloquent motivational speeches and so on.
Without him, then they will not have this "beautiful Arabic" speeches to motivate and recruit intelligent disaffected islamic people.
So we're left with the ones who make half-baked, clumsy rhetoric, who mostly only manage to recruit the stupider islamic people.
But those have been around for a long, long time. Society can handle them.
As for martyrs, well, many of the extremists go on and on about things such as the Crusades, so really, adding one more martyr won't make much of a difference.
And many of the islamic countries have of late been doing all that pro-democracy stuff, which further sidelines the extremists.
So threat of terrorism may go up in the short term, but I'd think it would decline long term, without a well-spoken smart man to make the speeches.