And for shorter periods (say, up to 25 years), you can expect to be pretty close to reality given the huge inertia of population aggregates. In the long run, we're all dead. John Maynard Keynes
I'm sure that Germany could manage to get a few hundred thousand net immigrants a year if some effort were to be put in, but it is not desired (for a variety of reasons, some of which are reasonable, some of which aren't).
And those are more common than one can think of.
Even the fact that we don't have a big war here since 45 is not enough: There was a demographic black swan in Portugal in 74 - The revolution made lots of people to return from Africa (here are talking hundreds of thousands in a country of 10 million - not negligible at all).
Outside of Europe we can still see big demographic movements caused by war (are you really sure we can ignore that happening again here?).
Also, resource constraints might make Europe unpalatable for more emigration (we are densely packed here).
And, by the way, Anglo-disease might the UK less competitive than other EU countries. I actually do believe that the forecast of the UK being the biggest country in population terms is utter bullocks precisely because of this.
