There was significant majority of non-German population in case of Reclaimed Lands to the Poland there were 5,1 million people of which only 500.000 were Germans and 4,6 million were either Poles or minorities close to Poles like Kaszubs or Silesians.
I don't have the exact data about East Prussia but around 1900 over half of population spoke Polish as first language and good chunk of rest Russian or Lithuanian. Later that % dimished significantly because of very active germanisation there. In 1944-45 population diminished severely because Germans and volksdeutches there got pumped condensated red scare and were running away like rats.

In short, on the western Reclaimed Land you would build same opressive entity you have now in Palestine and in East Prussia it would be hellish hot spot unless socialist and integrated into Warsaw pact, which was not an option west would accept.