Cultural homogeneity and qualities of values. People will not like hearing it until the day that someone could say "I told you so" once it is way too late, but here it is.
You will be able to trace the decline in Germany in particular as a kind of longitudinal study of what has long and thoroughly been established by science as the inherent conflict that comes from "diversity". You will be able to clearly track as the German culture and characteristics that made it "better than the rest of developed nations" decline as it is polluted with values and people that aren't just individually of objectively less advantageous quality, but the pollution aspect itself compounds those effects.
Essentially all research into the matter clearly shows that increased "diversity", combination of components of various incompatible and contradictory characteristics, means a fracturing and decline of social cohesion and focused interests in terms of human relations. It will be no exception with Germany, just the same way that you can trace how Poland and other eastern European countries that have not committed to cultural self-genocide and ethnic self-harm will not develop all the very problems that are already emerging and will only increase. This is not even a matter of just a European thing, it's a universal reality of human nature, when you combine substances, especially certain ones that trigger certain reactions, you will get those very predictable reactions; regardless of whether it's "diversification" of different people anywhere on the whole planet, just like if I force you to take in and support the livelihood of a drug addict, you will have some violent reaction to that. It's not really rocket science, yet for some reason the idiom, can't see the forest for all the trees, aptly and thoroughly applies.
Just like oil and water in and of themselves have certain desirable characteristics and are useful in and of themselves for different purposes, when you combine them you don't just pollute the water with oil and therefore make it useless, most, if not all people totally miss that in such a case you have thereby also polluted the oil with water and have made both substances unusable for their respective purposes. Additionally, the pollution aspect as also thereby accrued massive costs ... through a simple act of foolishness ... of separating and making the substances again valuable and of desirable and advantageous quality again that can be used for any number of relevant applications.