Is that a joke? There are many secular nations out there that have high standards for economy and quality of life. The Netherlands, France, Belgium, United Kingdom, Ireland, Sweden, Germany, Norway, Finland, Canada, Denmark, Malaysia, Israel, Iceland... and that is just off the top of my head. Sweden is the prime example, as it is the most rich, happiest, successful, well-educated country in the world, and also the most secular
I don't think you did your homework. In every single country you named the majority of the population (more than 50%) are members of one form of a religious organization or another. In Canada the Catholic school system is publicly funded through tax dollars (no other denominational schools are).
Since the topic is about religious symbols in Polish public institutions... in your "happiest" Sweden" over 70% of people belong to the Church of Sweden. Their flag depicts ... a cross. Look at other Scandinavian flags too.
Oh, and don't forget your very own British flag. It is a collage of crosses, and not just of any crosses but those associated with Catholic Saints. Yes, the good secular people of Britain wave Christian crosses on a daily basis, in and outside the UK. It is their tradition.
In Poland a cross has also to do with tradition and the Catholic religion has an important historic value. I gather your knowledge of Poland's history is pretty limited if you can't see it. If any symbol could be associated with the country's political roots then a cross would easily compete with the white eagle. Poland's statehood is counted since 966. Check on your own why.
However, I think Grzegorz's point was a little more poignant than that which has to do with economy and the alleged happiness resulting from it (is a bunch of LCD TV sets really a sign of happiness?)
This has to do with certain void that is starting to appear in some of the countries you named. That void is of spiritual nature and I believe that humans are spiritual, regardless of whether it is through organized religion or not. That void is easily filled by those who keep their faith, in this case Muslims. That is the only way I can explain to myself conversions to Islam by the "agnostic" citizens of Western and Northern EU. Imagine, how desperate for spirituality must someone be if the person is a woman and she converts to Islam, a religion that considers women of trivial worth.