Well, that's the question du jour, pun intended.
At some point they will have to or France will fall. Just as in a mob fight, if the initial combatants are not contained, then others will view it as license to join in. Then at some point there isn't enough organized resistance to overcome the mob.
In this case civil war would be the result. In many ways it can be argued that as close to a perfect society that is possible and that we've had, one characterized by a general peace since the last world war, is the undoing of those that would succomb to these muslim thugs. Many have gotten too soft IMO.
As undesireable as it may be to admit, someone's gonna have to decide that deaths to the insurgents are likely to occur if this situation is to be controlled. Then there will be an even greater response from the Muslim world which will need to be dealt with via an even larger blow to whatever contingent pursues that one. just as in all other wars, which this is, a war, one side wins when the other is subdued.
Muslims seem a lot more apt to be the subduers in these conflicts for whatever reasons and largely built upon teachings that this is their purpose in life which have likely been ingrained into them during their most impressionable years.
France's, our's, and the world's problem is that we somehow assume that it's our responsibility to see to it tha their militant and terroristic teachings are reversed by us and at our expense in spite of the fact they won't be no matter how hard we try. At least not en masse.