This particular operation is either ISIS-conducted or ISIS-oriented vigilanteism; whichever it is, backing down in Syria will only embolden them. (Or rather, embolden those like them; I don't imagine that very many of the specific attackers here are going to have particularly many opportunities to do this again in the future.)
ISIS is specifically out for either world empire or apocalyptic defeat (see http://www.theatlantic.com/magazine/archive/2015/03/what-isi... ); tactical concessions will work about as well as they did with the Nazis and the Communists -- or even less well than that, since neither Naziism nor Communism believed that success was a sign that Divine Providence was smiling on them.
You didn't study your history, comrade! Communist orthodoxy believed in a historical inevitability that overlapped very closely with a sort of man-made Divine Providence. The Nazis had their own set of religious and spiritual motivations, their racial destiny and so on.
They differed from Abrahamic religions in the sense that they expected "victory" in this world and in their time, as opposed to vague posthumous compensations and end-of-times prophecies; but they did believe in a "greater power" manifesting itself in their successful deeds.
> backing down in Syria will only embolden them.
That's a false dichotomy. The problems in Syria won't go away with bombs, and it was manifestly stupid for Hollande to join the party willy-nilly, especially after having experienced first-hand the inefficiency of his security apparatus. What is needed is a real agreement between the real power brokers (Turkey, Saudi, Russia, Iran) to cut off the crazies for good. We need hard diplomacy, not hard policies.