Figures of eight million French Muslims are regularly tossed around, based, it seems, on panicked fears of high Muslim immigration and a high Muslim birth rate. These figures are vastly overestimated, though. Figures on religious affiliation and ethnic background aren’t kept by the French government, as part of a long-standing reaction against the misuse of those figures by Vichy to deport immigrant Jews to the concentration camps. The suggestions of The Economist that there are a bit over four million French Muslims seem to be more sensible and generally accepted. This amounts to roughly 7% of the French population--a significant number, to be sure, but not an overwhelming majority.
If this minority population grew for the next 50 years at a rate of 2% per annum (a high rate, and one that doesn’t seem to be supported by signs of an ongoing demographic transition), while the remainder of the population shrunk at a rate of 0.5% per annum (also a high rate of decrease, and one that doesn’t seem likely to be achieved for a while given generally high French fertility rates), at the end of this 50 year period the total French population would have shrunk by 9%, and France’s Muslim population would amount to roughly one-fifth of the total. You’d have to wait for a century to approach a position of parity between the two populations, assuming the same unrealistic growth rates. This is definitely not any sort of imminent threat, nor as I shall demonstrate is it a very plausible threat at all.