Jonone, it's based on current birth trends, if they keep going at the same rate, which they are very likely too, and on the level of immigration into France from muslim countries. Countries have used these methods of anticipating future population levels, and the ratio of one ethnic group to another, for years, and they are generally always accurate.
I think the reason it is being banned, and in my view it should be banned here too, is because it is a disguise. I could go around wearing a hoody or a bandana or a tommy hat and they would all be fine, but if I chose to wear something which obscured my face then that would be differnt because I would not be able to be recognised. Do you think it should be acceptable for people to have passport photos taken whilst wearing crash helmets, or for people to stand in ID parades wearing ballaclava's? It would be crazy wouldn't it? As I said in an earlier comment, face to face contact and the expressions one conveys from their face have been a standard part of human interaction since human beings first existed - it seems ridiculous for that to be taken away. Imagine a society where EVERYONE wore a face disguise, it would be rather weird and people would only know who someone was by their voice - facial recognition would be a thing of the past. What a strange, and rather frightening, society that would be.
Not to the same extent. If I saw someone tomorrow who I hadn't seen for a year and he had a beard, something he didn't have last time, I would still know it's him - same with a pair of sunglasses on. I could also talk to him and read what his mood was, and in what context his utterings were meant, by the expressions on his face that would come with the words. If he stood there with a sack over his head with just his eyes visible then I would have no idea it was him, other than that I could recognise his voice, and I would not get the expressions along with the words that would add dimension to what he is saying.