Marseilles is roughly 30% Muslim. I don't know the stats for Nice, but as Nice is nearby, it's reasonable to assume a similar population.
The Muslims in France are having babies. The French are not. That means that the newborns in Marseilles and Nice are probably split about 50-50. You can't have any more 21-year-olds in 20 years than you have 1-year-olds today. Marseilles and Nice are within sight of Muslim parity and then majority.
Modern warfare in the West is fought at the ballot box. A voting majority can impose their will on the rest of the population like an invading army. I'm sure there are constitutional limits to what elected officials can do in France, but they exist here, too, and they appear pretty easy to brush aside. Amendments 1, 2 and 5 are all threatened these days and not by wild-eyed radicals. The California constitution briefly included a definition of marriage that was struck down by judicial whim. In the post-modern, secular West, a constitution doesn't provide much protection when the majority wants to exert its power.
If I was a French father and husband in Marseilles or Nice, I'd be seriously looking at moving north. You make different calculations when you've got a family to protect than you do when you're a member of the political, artistic or academic elite. They can talk and talk and talk about multiculturalism and lone wolves and legal processes, but when it's your wife and children, those words mean nothing. That 50-50 split mentioned above isn't going to take 20 years if the French start fleeing north.
I can't think of a single place where the Muslims have a majority that hasn't established some form of Sharia. I wouldn't be surprised if there were some, I just can't think of them. That's where Marseilles and Nice are headed right now. If yesterday's attack starts French flight out of those cities, the Caliphate is coming to southern France sooner rather than later. In any case, it's coming eventually.