Borjas does not make ethical arguments or attempt to tell people what they should do, he only lays out the choices and some possible answers.
If you start from the assumption that everyone in the world is born equal and all deserve equal opportunities then the correct solution is to drop your nation's borders entirely. No borders at all, anyone can come. Germany basically tried that and lasted less than a year before the native population started to revolt. It's now stuck in a kind of ethical limboland where theoretically Germany still welcomes any "refugees" but is spared from actually doing so by other countries. I put refugees in quotes because there's no way to reliably distinguish refugees from people who just want a better life by the time they make it to Germany: they arrive without papers or proofs of any kind.
The reason the native population of Germans were unhappy about this policy is fairly simple: there's a perception that some countries are objectively poorer and worse than others because the people living there collectively made bad choices and built bad cultures. Thus importing them risks importing the problems of their homeland and killing the goose that lays the golden eggs.
For instance, India and South Korea both started out the 20th century being extremely poor and dominated by foreign invaders. They achieved full independence around the same time. By the end of the 20th century South Korea was an extremely sophisticated first world nation (this is the "miracle on the han river") but India was still very poor. Would letting all of India into western nations make everyone as rich as Americans? Or would it just flood native culture and decision making with caste politics and other rather non-American value systems?