The mistake was owning slaves in the first place. Enlightenment values gradually caused people to hate slavery over a few centuries, and once it was abolished, people then failed to recognize outside ethnic groups as invaders for the first time in history. After another century of social justice activism, they warmed up to the idea of allowing "refugees" to enter their countries, and social justice has continued to the point of white ethnic masochism becoming the norm like it is today.

That's what's so dangerous about progressivism, it's paved with so many good intentions, but it all leads to the situation of today. It's really the ultimate slippery slope, nothing seems wrong until it has gotten to a point where our very existence is at jeopardy. As such, the better solution would've been to never own slaves, just keep everyone else out the same way that every other country on Earth does.