Earlier quoted context omitted.
I'd like to see a worldwide agreement that no country will ever stop a citizen from leaving, as long as they have permission to enter another country. Even if the person is in prison or debt, they can always leave with just themselves to another country willing to take them. If all nations agreed to that, we could stop worrying about most other human rights abuses, and country governance becomes more of a free market…
The free market has a pretty terrible track record when it comes to human rights, though. What would countries stand to gain from this, realistically? No country wants economic refugees.
The main reason not to take migrants is you might see substantial economic losses as migrants send income back home to support family. You might alienate your existing population with conflicting cultures. And with enough immigration, the migrants might eventually outnumber the original population and persecute them (eg. indian reservations).
It's all a tradeoff, but I suspect there always a few countries worldwide who will take anyone.