Why do European countries need immigrants while Japan does not?

This is a serious question, and I do not want some reason claiming the economy will fall apart if immigration does not exist, because as the United Nations and OECD have reported "There is no evidence that immigration is a solution of population problems, economic growth and long-term lack of labor."

The questions must be asked: Why are European countries bullied into allowing not only immigration, but MASSIVE immigration into their countries? Why is every single Polish person living in Poland wanting a better life and wanting to leave the country, yet immigration has picked up from hundreds a year to over 10,000 a year in just the last 3 years? Why is Japan not being bullied into allowing immigration? Why has Japan's economy not collapsed although they are losing population? Why has the Czech Republic increased immigration from a trickle to 30,000 a year for the past decade?

Genocide?

"Generally speaking, genocide does not necessarily mean the immediate destruction of a nation, except when accomplished by mass killings of all members of a nation. It is intended rather to signify a coordinated plan of different actions aiming at the destruction of essential foundations of the life of national groups, with the aim of annihilating the groups themselves. The objectives of such a plan would be the disintegration of the political and social institutions, of culture, language, national feelings, religion, and the economic existence of national groups, and the destruction of the personal security, liberty, health, dignity, and even the lives of the individuals belonging to such groups."

-Raphael Lemkin, Jewish man whom coined the term genocide.