The problem is that it's not the third world's population should go to first-world countries. It's first-world's legal and economic (and associated societal) technology should extend to the third world.

Third world's elites fear it because it will render most of them irrelevant (and it will). Third world's nationalists and leftists fear it because it will change the local culture to look like the fat bourgeois western culture (and it will).

The previous foray into this was tried at the times of colonialism; the results are ... mixed even by the most favorable account.