They have been christian for over 2000 years.Very few native europpeans will become muslim,this might escalate into something similar to the reconquista war if people get radicalized.This is one big problem people are pretending not to see.
Even average White Nationalist clearly have no appetite for Christianity. And not even talking about the French yett who have strong laïcité culture.Muslims too divided on secterianisms and ethnicity and not counting the booming apostasy rate from them too and thus have no way to hold any real leverage on realpolitik. Immigrants always at the mercy of the government
Society in 50+ years will be a lot different and will be a lot more militarized and thus upholding current status Quo and the current political technology thus any tangible change is unlikely
Reconquista is a lot more nuanced than people like to think.