I advocate for almost completely open borders, for three reasons, freedom + economic growth + national security.

(1) Freedom. Closed borders was actually one of the first iterations of technology being used to suppress human freedom. We had open borders for most of human history. Only with the advent of modern nation states (and the associated mind virus of nationalism) and recent technical capabilities have we both been motivated and capable of blocking people from travelling and moving freely.

(2) Economic growth. Self-explanatory. Look at where our top founders, or their recent ancestors, come from. The more immigration, the better.

(3) In the long-run steady state, geopolitical power comes from population size. China and India surpassing the US economically and militarily is a near inevitability and a matter of time. I'd rather a liberal democracy play the role of world police than an authoritarian regime. But that's only possible with 600 million+ Americans.

TLDR, closed borders is a disgusting modern aberration with racist and protectionist motivations that stunts the growth of countries and eliminates the freedoms of humans.

That analogy may be more insightful than you meant it to be.

Animal cells (e.g., the ones in your body or mine) do not have cell walls. They do have membranes surrounding the cells, but they aren't walls.

In the same way, any country has borders, but there's a lot of scope for variation in what they allow across the border under what conditions. "Open borders" does not mean "no borders".

For instance, the borders between countries in the Schengen Area of Europe are "open", even though those countries still have borders. This has been the case since 1995. Civilization there does not appear to have fallen so far.

Pre-modern world having no borders is what gave birth to immigration into the said country & other parts of the world which got colonized over time. Kinda hypocritical to suddenly have everything closed up. Who knows maybe space travel or resource scarcity is what unites all countries centuries down the line.