Fundamentally the French economy isn't producing enough to support the current level of pension spending, due to a continuously falling ratio of workers to retirees.
Of course an influx of immigrants will cause other issues. But if wealthy people need more population to keep their wealth up, I don't think they care.
It's part of the official discourse.
France overspent in the 2000s and 2010s due to populist politics, and now the chickens have come to roost. Something needs to give in France, otherwise it'll become an Italy 2.0.
I mean, try it with yourself. Imagine that you take your current salary, keep it for 10 years, and have 100% of your spending covered? How life changing would that be to you? Probably a lot.
Understand that the non-wealthy wouldn't get anything new, it would just be a continuance of their current services and benefits. They've have 10 years to increase their GDP by 50% to get the tax revenue to get to a balanced budget. That's simply not happening without obscene inflation, with the corresponding increase in government spending on goods and services, keeping them out of whack.