Lily, not trying to be mean, but open or concealed carry is not going to matter much. A smaller armed force can destroy, decimate or otherwise scatter a larger armed force if it has ambushed from a superior position.
Case in point, one motivated guy with a rifle shot over a dozen openly carrying police officers in Dallas who were on heightened alert.
He had the advantage of weapon type, surprise, cover and concealment and height.
The Texas University Tower shooter, Whitman, shot most of his targets in the first few minutes and even though some citizens got their own rifles and shot back this only confined him to one corner of the tower, where he still had a commanding view. It took close quarters assault to take him down.
The only warning the muslims would give a crowd is “Allahu Ackbar!”
The only salvation of being armed in a situation like this, if you are not already killed or incapacitated, is that you can now fire back. But that is shooting back at an enemy that may hold tactical superiority and is not afraid to die, even believing that he is already dead. You have to decipher how many, what type, and roughly what skill level the shooters have, and you have no idea how much ammo they brought to the party. It’s best to assume more than you.
Here is what I would suggest to the French People. Arm yourselves and rather than wait for them to attack again, root them out and deport them at gunpoint. No muslim, no problem.
An armed citizenry is important. But as you noted, it is of limited value. For example, armed citizens could not have prevented the Boston Marathon bombing.