Agree with open borders part. At the same time you can't keep moving to new territories forever. Sooner or later you'll fall off the edge of the Earth.

Sometimes borders guarded by people with guns are there for valid historic reasons. Maybe those economists are thinking too theoretically and shallowly, and overlooking the unfortunate but very real complexity of life.

Funny, that was a (highly criticized) solution used by the US when it comes to oil. Go in and remove the people with the guns - using (or buying) force when economics fails. And economic means fail quite often - people are not always rational actors who do the things that are in their best interest.