[AdrianB1]

From an economic perspective the ideal is no borders if there are no significant differences between countries that would create an infinite surge in mobility. It's like electrical current, if there is zero resistance and a difference in potential, any short circuit will potentially destroy the entire circuit.

[wizzwizz4]

The "infinite surge in mobility" phenomenon only occurs if we model countries as infinite sources / sinks of people, and assume population movement has no impact on either country. Given both of these assumptions, the predicted phenomenon wouldn't cause any problems. Of course, neither assumption holds in real life; and if you re-do your models with more sensible assumptions, the phenomenon goes away.