Cultural homogeneity and qualities of values. People will not like hearing it until the day that someone has to 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 over time as the German culture and characteristics that made it "better than the rest of developed nations" decline as it is corrupted by values that aren't just individually of objectively less advantageous quality, but the pollution aspect itself compounds those effects.

Essentially all research into the matter of human social conflict clearly shows that increased "diversity", combination of components of various incompatible and contradictory characteristics, means a fracturing and decline of social cohesion and focusing of interests in terms of human relations. It will be no exception with Germany as it has crossed the Rubicon, just the same way that you can trace how Poland and other eastern European countries that have not committed to cultural self-genocide and diversification self-harm will not develop all the very problems that are already emerging in Germany and western Europe in general and will only increase going forward.

This is not even really just a European matter, it's a universal reality of human nature. When you combine things against their nature or will, especially certain ones that trigger reactions of varying violent degrees, you will get those very predictable reactions just like any chemist will tell you. That violent reaction will be triggered when volatile substances are forced to combine, regardless of whether the motivation is "diversification" or not. Violating people's dignity, rights, and presumed democratic self-determination will result in violent reaction, regardless of any other desired outcomes. Throw potassium in water and no matter how hard you hope and dream, the reaction will be predictably violent and forceful. Democracy is under threat, ironically from people who think they are doing good by violating people's rights and deconstructing democracy in the most fundamental manner. Just like if I force you to take in and support the livelihood of a drug addict, you would have a violent reaction, so inevitably will the current violations of human dignity cause a violent reaction. 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.