My thinking is precisely the opposite.
These "kidnap & behead" operations make it eminently clear that they have nothing left, and that they cannot/will not confront the enemy directly. Beheading unarmed innocents does not equate to "heroic jihadi martyr," it equates to "bestial savage." It does not reflect well on the people of Islam.
It's quite clear that Islamist operators in Muslim countries are rethinking anonymous and indiscriminate bombings, because those kill huge percentages of Muslims rather than infidels.
I'm still trying to put some articulation onto the notion that "capture & behead" is qualitively more disgusting and ignoble than a car bombing, even to Muslims. But it is, and I think this is going to backfire on the jihadi cowards.
My best musing hopes that these little ploys will overplay the hand of the terrorists, but I doubt it. Like self-destructive dysfunctionals in any family, they want a self-fulfilling prophecy to prove them right: they are oppressed and persecuted. Never mind how they got there. If we press our righteous case against them, we are the bad guys. A very big part of Al-Q wants the world to "bring it on." They have little else to live for.