Mosque attendance is also dropping.
Especially in the lowland countries, the numbers of Muslims leaving is large.
There is a general large exodus to atheism across various religions - whether the “flavours” of Christianity or Islam or Judaism or hinduism or Sikhiism etc.
To put this in perspective, the immigration problem is a problem because they don't integrate. Whether the principle of association is ethnic or religious I'll leave to you to debate with someone else. It is sobering enough to see that those who do not leave the mosques are more devout than ever.