The migrant crisis is merely a symptom of what is killing Europe; the cause is liberal-plutocracy.
As you've suggested, Poland is mainly a victim of emigration in this regard, not immigration. Millions upon millions of Poles have been lost to Poland as a result of the allure of the Anglo centers of capital, America and Britain. In fact, more Poles than were killed in WWII have left Poland, and most of them will never return! This is well known feature of liberal-plutocracy: brain-drain, or a sort of inverse imperialism.
For example, the reason why I can hardly call myself Polish and have the pan-European views that I do is because my parents emigrated to America and sacrificed the Polish identity of their progeny! My sister married a German-American (ironically), and I am an incel who has been left to cope as best I can. But even if I were a normalfag, I wouldn't be able to marry a Polish woman here but rather some American mutt! I wish it didn't have to be this way, but I have been completely deracinated from my homeland (which I could've been of tremendous benefit to, as my ancestors were).