The way I see it is the cultural change happened after the Prussian elites lost power and the general German public understood their recent history as far from the eternal victory they were promised. That's a major cultural change however you want to spin it, and there are enough examples in history where countries haven't gone through the same changes in similar situations (Weimar Republic also comes to mind)
Regarding the current day pacifist state of Germany, in my mind it is still very drastic anti-militaristic compared to the vast majority of the world.
Regarding corruption, countries changed drastically during the 20th century due to ideologies and individual action, sometimes resulting in disaster but also often not. Trying to say nothing can change is a defeatist deterministic attitude that has no anchor in recent history.
Presuming we are talking about migration, most of these source countries were able to fight and win European colonization in the past, why do you think that's easier than fighting corruption?