One of the fundamental functions of a country is to regulate input/outputs at the border.
There are so many disastrous problems that arise from the elimination of a border, that I question whether such a suggestion is ever even made seriously or made by sane people.
(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.
1. the us had open borders because they could offer land to people coming in, it had economic benefits for everyone involved. nowadays, EVERYONE wants to get in, especially actors with bad intentions, and as was just shown in europe not even five years ago now, open borders are a terrible idea with long lasting consequences.
2. economic growth does not simply derive from a larger population, there's a balance between population size and the economic prosperity of each individual within that population, to tip the scale so absolutely in only one direction completely destroys the scale itself
3. geopolitics are not so black and white. it derives complexities in more areas than just population, and the US does not have a small population, nor do its allies
for the past few thousand years, you risked your life by traveling. if someone went around looking for slaves to sell and you found yourself clamped in iron, your life was over
Of course closed borders are protectionist. I don't think that's a bad thing.
I'm making practical and utility arguments here but I also simply think noone has the right to prevent someone's freedom of movement unless the stakes are truly dire (infectious disease with high mortality). I believe it's evil for people whose ancestors immigrated (which is everyone, including England, which used to be black inhabited) to then point guns at the border and prevent others from doing so for no reason whatsoever aside from fear and bigotry masquerading as vacuous excuses. The planet is so small, it's a heinous assault of freedom to deprive humans of the freedom to explore and settle around it during our brief existence.
Due to the lack of a good alternative, we allow anyone born in this country to be a citizen, and to vote at age 18. I wouldn't want random people who don't speak the language nor have any interest in sharing our culture or respect for human rights or democracy to have political power. (See: the importance of public education for a well-run society).
So let's say we allow an unlimited number of people in with almost no barrier, who may or may not share our political values or language, to live and work here. Sure, it's happened before in mass immigration movements. What if we said, "You can be here, but you can't vote until you are documented to have lived here for a certain period of time and can pass a citizenship test?"
That may work, but I wonder what will happen if we continue to have (as we now do) a two-tiered system ala Starship Troopers; one of citizens and one of "civilians" who do do not have political power. What are the impacts? I am not sure.
My core belief is that education and common values are important to a society, and I am concerned that unfettered immigration into this country would risk destroying whatever common culture/language we have over time.
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.
(I don't actually think the details of what cell walls, cell membranes, skin, etc., do and don't regulate / completely block really tell us anything much about what should happen at national borders. If tomorrow someone discovered a previously unknown regulatory mechanism in cell membranes, that shouldn't make anyone change their mind about immigration controls. The way these analogies actually work is that someone decides what they think about immigration on other grounds, and then goes looking for an analogy that fits the rhetorical point they want to make. Someone arguing for "open borders" would probably choose something else rather than cells to make analogies with. The analogy isn't doing any real intellectual work, it's just window-dressing.)
A membrane or wall regulate the input/output of everything in/out of the cell - from water to proteins to everything else.
A cell without a membrane dies - immediately.
You had tried to support that claim by making an analogy with biological cells, and it pleased me that in fact my counter-point fits into that analogy, but it doesn't really matter: if it turns out that the cell-analogue of "every country has to have immigration controls at its border" is true, all that means is that cells were a bad analogy because, again, it is simply not true that every country needs immigration controls at its border.
I am not a cell biologist so this might be wrong, but my understanding is that cell membranes are in fact permeable to water and don't have any particular ability to regulate the movement of water in and out of the cell. The rate at which that happens will be affected by e.g. concentration of solutes on each side, but it's not a thing a typical cell has mechanisms for controlling. I think there are such mechanisms in some cells in the kidney, but AIUI while most (all?) cells have aquaporins there's nothing adjusting their permeability. But, once again, while this is all interesting in its own right it tells us nothing about what countries can, or do, or should regulate. If some researcher makes a stunning discovery that overturns our understanding of what cell membranes do, the appropriate adjustment in national-border regulation will be zero.