Not a bad take overall honestly. However, an important piece you may be missing is that the federal government should have enough power to prohibit states from engaging in practices that are clearly beyond the pale of human decency. Like slavery. And, I would argue, banning gay marriage. We don’t need mini “laboratories of democracy” to resolve those questions anymore.
Even in the 21st century, not everyone can simply move to flee oppression as easily as you think. People are rooted to their homes for all sorts of complicated reasons — jobs, family, school.