What's wrong with non-European foreigners gradually replacing Europeans in England? You don't complain about the Europeans replacing the Native American in the Americas or the Europeans replacing the aboriginals in Australia.
You ask why there is reduced assimilation. Are you sure there really is reduced assimilation? A fair comparison would compare first generation to first generation, take into account lengths of time after immigration. Makes no sense comparing first generations who have immigrated for 20 years to third generations whose families have migrated 80 years ago.