But scientific evidence shows that this leads to inbreeding and serious social problems, and there are numerous examples of this.

Eugenics is against inbreeding. Inbreeding is usually caused by cousin marriage in real world populations and has nothing to do with eugenics outside some retarded strawman you've probably concocted in your head.

that the mixing of people has made them smarter, more resiliant, and more attractive

Did it? Funny all the mixed people I live near are uglier, stupider, and more impulsively violent than their unfortunate monoracial ancestors. I guess their IQ scores, crime statistics, and mirrors are lying about it though. I can't think of a single result of mixing European and other racial/ethnic groups where the average was an improvement.

Here's a liberal science devotee saying the obvious though https://twitter.com/richarddawkins/status/1228943686953664512?lang=en