I agree with you on demographics, but disagree on your explanation.

(Although I'm curious what mechanism you posit between Islamic migrants and less nudism...)

I think the demographics explanation that really works is that nudism as a philosophy belongs to an older generation.

I was raised by nudists, and spent many an awkward holiday in various nudist clubs or camps as a child, and the customer base was already dominated by older people back then.

A lot of the for-profit nudist colonies have closed down or shifted to semi-nude, because there's so much less demand.

But don't worry, the dedicated nudists still get nude, they form clubs and rent out private camps.