I think mixing human cultures can be very complex (human social culture is certainly some form of a complex system) and you're correct that it can cause a lot of change and possibly danger like culture getting erased. When cultural integration is done well, it can cause a lot of beautiful things to happen: appreciation and love of the culture(s), sharing the culture, and fusions of culture. Please note: The above is a lot harder said than done.

Where on earth is this actually happening? It's a pipe dream. It's absurd.

Because from where I'm sitting, with this current global trend of mass immigration, we are seeing - in every single case - the loss of high-trust cooperating populations of ethnically homogeneous people to ruinous hellscapes of filth, violence, conflict, etc. Lowered quality of life in almost all areas. The destruction of ancient homelands of indigenous peoples to temporarily prop up GDP.

I'm sick of pretending otherwise. Does anyone seriously think that Japan would be the same place if every ethnically Japanese person died out to be replaced by millions of Indian or Sudanese immigrants?

All throughout history, when multiple ethnic groups converge in the same place it ends in violent conflict. No exceptions. Not ever, evidently not today.

You really think this is a modern “problem” (if a problem at all)?All empires throughout history (Egyptian, Persian, Roman) left deep cultural changes that still can be felt today. Thinking that only an “economic globalization” without people moving to more prosperous lands could occur is the pipe dream.