You make it seem as that, without any immigration, France would be some sort of European backwater, if not economically at the very least culturally? This is 100% wrong? Let us not forget stuff like the events of 1968, or really the fact that France has a long and very vibrant culture?
And even if it was true, would it actually be bad? Switzerland is (stereotypically at least) a rather small c conservative country, yet it seems to be doing pretty good to me?
Which is the things, France has a long a succesful and vibrant culture which hit a high point during the trente-glorieuses, but has been unable to move on since. So in the arts, or food, or whatever; alot of it is just trying to reproduce the golden ages and the most innovative stuff in cinema, music, art, comedy, whatever is done by people with a migrant background. In that respect, the point I was making is that yes, immigration has "enriched" France culturally - and has in Switzerland too; because people have always complained about the small minded, conservative mindset here which has only started to move on as people have become more open since the 90s.
The ship for that scenario has long sailed of course; the "mistake" was done in the 1950s and not now; and hindsight is 20/20, especially 60-70 years from the future; so this is just a thought experiment and an irrelevant question in 2020.
As for the rest, being a muslim, even a relatively religious one is not disqualifying from mainstream (French) society. But unfortunately being religiously muslim does tend to be heavily correlated to other beliefs that are disqualifying from mainstream society. Most muslims do not have said attitudes; but it is a percentage that is a lot larger than it should. Plus of course there is the huge issue of segregation where there are communities where you could easily live while speaking just broken French. There is a reason that media often puts fear about "no-go zones". Even when they are relatively safe, they are still a parallel society with little mixing with the rest of France.
I am actually not sure if France has this kind of area and France does not collect ethnic statatistics anyways, but think of a place like Rinkeby in Sweden, where 90% of the population is of immigrant descent. Though from what I've heard Seine-Sant Denis seems to be the equivalent to a place like that?
This isn't actually that true - there are tonnes of statistics and studies out there that, even if they can't directly survey people about their ethnicity, can infer it. So you have studies that show that a person with a Muslim sounding name would have to send out ~30% more CVs before getting a response as an equally qualified person with a French sounding name; there are studies that show that living in an area with the "wrong" postcode can have the same effect; multiple studies showing how certain quartiers are enclaved in a way that practically cuts them off from society. And just anecdotally, merely appearing in the media with a headscarf is a more or less guaranteed way to create a media shďtstorm and have people sending you death threats. What kind of a message do you think that sends to someone who is Muslim about how accepted they are?
There is, unfortunately, basically no way of arguing against the fact that being Muslim, or even appearing Muslim, is going to leave you facing a certain level of social rejection.
As for the certain communities, thing, well that is urban planning choices about enclaving certain communities, rotting social housing, lack of transport links, lack of social services that contribute to a feeling of rejection. There is nothing new here.
The political will in recent times has been that any problem in say, the cité des 4000 in La Courneuve (a crumbling 60s housing block that is a quite literal island surrounded on all sides by motorways), has been to send in the police. At the same time as cutting social services, firing social workers, closing youth clubs and so on. The results have been predictable, because you ignore an emerging problem, cancel all the possible ways of fixing it when you still can, and you end up sending in the police to do the jobs of social workers after it's too late - made all the worse by the fact that the French police has an issue with racism that has been documented endlessely over the summer.
Like this is the thing, if they are parallel societies it is because they have been built in a way that makes them actually physically separate from the rest of society; have then born the brunt of the retreat of the welafre state and their residents now face an intense level of social stigma just because of which neighbourhood they come from. This isn't just a mistake made in the the 1950s, it is a mistake that is continuing to be made today, because the solutions are too complicated, too expensive and too unpopular.
Even then, Seine-Saint-Denis is a much more complex and mixed community than people think from the outside. It's not just 60s grands ensembles, drug dealers and gangsters like people would think. Allegedly "no go areas" like Saint-Denis or Aubervilliers are currently in the process of gentrifying (Montreuil is basically already there); places like Aulnay-sous-Bois have big Indian, Chinese, East European communities as well as "ethnic" French.