I wanted to spend a few years working abroad to learn other cultures. I was not able to do so because the countries I wanted to visit would not give me a work permit.

It has traditionally been easier to get a work permit in the US than in other countries. I suspect that, in a few years, it will become impossible for people who are in the United States to leave the US.

There was a study done last century. The study discovered that most people (like 95%) never traveled more than twenty miles from their home in their life. Mass migration was not an integral part of human experience.

The world travels that you read about in history almost always involved state sponsorship and the people who engaged in the travel were protected by armed guards.

As for the United States. The US broke the mold of highly restrictive travel.

If we adopted the Marxian system, we would probably see a world where all boarders were closed and travel dictated by the state.