YOU FAIL AT EXTRAPOLATION ******.
An eloquent start. Shall we dance?
-Firstly any statistic gathered on French religion is slightly suspect, as the French government specifically forbids religion on census forms so its normally some private company doing the data collection.
Yes private companies are all evil, wot wot, but I'm pretty sure that polling sources are valid. Demographic statistics ain't exclusively the purview of the French census bureau, you know.
Even so they generally have people claiming to be RC (generally a holidays and major events attendance rather than weekly) at ~50% compared to Muslim 4%. Saying the French all appear secular is like saying all Americans live in California or New York because that's all you see in the movies - what you see in the media is not representative of the nation as a whole.
I've been to France three times, first off, so your point that I'm an uninformed, media-swallowing American falls flat there. The French appear secular because they are, for the most part, secular.

Now, to the statistics. Approximately only half of those 50% claiming to be Roman Catholic actually
believe in God, so the Catholicism is more 'cultural heritage' than legitimate practice. The number of atheists and theists in France are roughly equal, and since a large portion of those theists are non-Catholic, the number of French atheists FAR outnumbers the total of French practicing Catholics.
-After 60 years of Muslim influx, they compose about 6-8% of the French population, and France is trying to restrict the number of new immigrants. Even if you give the current Muslim population something insane like Chad's growth rate (2.5%) and have the non-Muslim population falling at 1% a year (5 times worse than Russia's!) after 50 years of that the Muslims only make up 48%! If you use something more reasonable (but still incorrect) growth rates like Tunisia's rate for the Muslims and Russia's for the non-Muslims they'll be standing tall at 17% in 50 years. Yes they will have increasing influence, influence that in 50 years will still be much less demographically powerful than the Christian Right in the US is now. This is without addressing the 'all Islamics operate as an unchanging hive mind' thing.
Actually, you make a good point. But where exactly did I say that the Muslim population would overtake the Caucasian French one? Oh right,
nowhere. My exact words seem to be "political overhaul" and "theocratic playground". Particularly in Paris, where a large concentration of poor Arabs live, the ability to cause violent chaos rises with the number of impoverished, radicalized immigrants moving to the area. I'm not making some sort of racist, anti-Arab statement, nor am I saying that France will be replaced by a Muslim state. So, you appear to have incorrectly extrapolated my argument.
What I WAS trying to state was that since the birthrate of non-natives and their descendants is significantly higher than the norm, it's only a matter of time before those of Muslim descent compose a more statistically significant portion of the French population than their current ~7%. This increases the potential for radical Islam entering the political sphere, further incitements to violence, terrorist actions, and
There is nothing inherently bad about Islam in France. But the chances for religious-inspired violence (eg. Antisemitism etc.) become greater with the rise in Muslim population.