> In the US, over 3 million Puerto Ricans are now allowed to migrate to the mainland US (say to DC). The borders are completely open to them. In Europe, similarly, around 20M Romanians are allowed to Sweden. But they don’t

According to Wikipedia, as of 2018 there were 5.8 million Puerto Ricans living stateside. The population of Puerto Rico is around 2.8 million. There are more than twice as many Puerto Ricans living on the mainland as there are in Puerto Rico.