Do you recognize that your argument can be applied to literally any culture that wants to ethnically preserve itself (just replace "white" with "Japanese" or "Chinese" or "Nigerian" or whatever)? You are arguing that any civilization that wants to preserve its ethnic character is fundamentally illegitimate, which is ironic because that view is at root a direct outgrowth of the European universalist enlightenment tradition. You think you are better than other cultures, and you are using that to self-righteously argue for their destruction.

My argument is contingent on the idea that behavior/personality is somewhat heritable, and that culture and ethnicity co-evolve to a certain extent - statistically, people are more likely to be better suited to their own culture. Finns are better at being Finns than they are at being Italians, and vice versa.