The population of western europe is decreasing for the same reason the white population of the usa is.
People(women) are too worried about getting ahead and establishing their careers and putting off childbearing until they can afford it.
I have been eurotrash now for nearly 20 years and all that welfare is to try to give people some security so they will have kids. It appears that it is working in Norway where birthrates are replacement. If you are sitting in an unemployment line who wants a house full of kids to feed to add to your woes.
A very practical reason...like it or not.