Here's a thought: if you were a 45 year old working in France today and you were looking forward to retiring in 15 years, would you be worried that a bloc of young, Muslim voters might decide that they don't want to pay all those taxes to fund pensions for those who look down their noses at them? The French pension system is not a savings system, but one that is funded by current taxes. And, few people have non-state pensions.
This goes for the entire social security/welfare system in France. Younger Muslims might recognize that they can better take care of their own and vote accordingly. They don't have the economic clout to do this yet, but it is inevitable that they will grow increasingly strong economically given France's aging non-Muslim population.