None of the science experts you're hearing from has ever taken a course in ecology and evolution, or a general course in mathematical biology. (And if they did, it sure didn't leave an impression on them.)
This is crucial to keep in mind when you hear them analyze what is currently going on with the coronavirus pandemic, or when they describe what they think are the likely outcomes of various possible decisions that we could make. Most of their training is highly specialized, the most lucrative careers are in the micro rather than macro level, and therefore their views are rarely informed by holistic frameworks like ecology and evolution.
For example, an ecologist knows that when a set of pressures (or "incentives") continue to exist, the roles that they select for will continue to be filled. In a virgin niche with abundant resources, somebody is going to start gobbling them up. That first lone organism (if we're comparing different organisms), or that first species (if we're comparing different species), is like an actor playing a role.
The role is written there in the screenplay -- hero, villain, etc. If the finale involves a clash between hero and villain -- some actor is going to perform the good role, some other actor is going to perform the bad role, and their interactions will play out in a spectacle of good vs. evil, no matter which particular actors are in the roles.
The role is a variable rather than a specific number. It is an abstract concept, not the particular thing out there in reality that behaves according to its traits and rules. Interactions between roles are also an abstract concept -- predators vs. prey, hosts vs. pathogens, sick vs. infected vs. recovered, etc. -- not the particular individuals or species that interact in such a way in reality (lions vs. zebras, or whatever).
In fact, "epidemic / pandemic disease" is a variable, since there are all sorts of pathogens out there that could play such a role. When we're talking about the coronavirus, we ought to emphasize that we want to prevent all such nasty pandemic diseases from spreading -- not just snuff out this specific one. Who cares if we magically eradicated COVID-19, only to be wiped out a few years down the line by some other nasty pathogen?
In short, we want to eliminate a certain role from the screenplay altogether, not just lobby for some particular actor to be fired from the role (and implicitly, for some other actor to take their place).
We must therefore analyze the current pandemic in abstract terms, and make decisions that will prevent similar pandemics from occurring in the future. This also means looking over the history of epidemic / pandemic disease to discover similar environments to our own -- and environments that were spared of pandemics -- in order to abstract away from the particulars and figure out some general principles, which can then be applied in the future.
As covered in the recent summary post on the science and history of pandemic disease, the main factor is demographic interconnectedness across many different population clusters. In other words, open borders. It doesn't matter what's causing this linkage network across various groups -- trade routes, military expansion, mass migration, etc. They all serve to spread germs far and wide, and crucially to ramp up the population size in which the germs are spreading, because several populations are being combined into a single meta-population.
The larger the population size (really, the density), the easier it is for pathogens to explode in epidemics, instead of quickly snuffing themselves out. That's why hunter-gatherers were never stricken by crowd diseases like measles or the flu, which only emerged in the large (and dense) populations of sedentary agricultural societies.
So, what happens if by some miracle we eradicated COVID-19, the specific disease afflicting the world right now, without altering the ecological structures and pressures that gave rise to this pandemic in the first place? Obviously, some other pathogen will step in to do the job -- if you fire one actor from a role, some other actor will end up in that role, performing the same behavior that is called for in the screenplay.
Of course there may be a delay, the next one might not step in right away. And of course, they could end up having their own different take or interpretation on the role, no two actors will perform a role exactly the same way. But the point is, if the role is written into the screenplay, sooner or later, and somehow or other, it's going to be performed.
If you eliminate the current predator of some prey species, then some other species will take their place as predator. Sooner or later. And perhaps with their own different take on the role (maybe they're more vicious than the last predator, or maybe less vicious -- but still predators against the prey).
Therefore, if we eradicated COVID-19 (big IF), but then went back to the demographic flow between populations that we had before it emerged, we would simply set ourselves up for another pandemic. Maybe it would be another influenza-like bug, or maybe it would be from some entirely different class of pathogens. There are so many aspiring actors just champing at the bit to play this breakout role! Thus, our solution to the current pandemic must minimize demographic flow between groups.
Then there is the evolutionary angle -- suppose there were no other pathogens out there to step in and take the role from COVID-19. It's the only actor possible to hire for the role. But we somehow block it from playing that role -- we isolate foreigners, practice some degree of social distancing, trace contacts of those who were infected and quarantine them, maybe administer a vaccine or other treatment, and so on and so forth. Problem solved, right?
Wrong! Much like a frustrated but talented actor would do if they got rejected after the first audition, they would do their damnedest to change in order to adapt to what the casting director wanted. Dye their hair, bulk up their physique, speak with a different accent, go more method-acting, whatever it took. So will COVID-19 mutate and evolve in order to adapt to the barriers we try to place between it and us.
Suppose we figured out empirically that we needed to keep a 6-foot distance from other people, and that was all it took. No further lockdowns or anything -- just stay 6 feet away from others. What if one of the individual viruses has a genetic mutation that allows it to spread at a distance of 10 feet? Then just staying 6 feet away won't contain the disease from spreading, and it'll spread. Only now it will be the descendants of that one individual with the mutation that allowed it to spread at a somewhat further distance. But we'll still be stricken by a pandemic nevertheless.
Then we'd get into an evolutionary arms race. We would not have much time to evolve genetic defenses against it in the short term, unlike the virus whose generational time scale is far faster than ours. But we could evolve cultural defenses (social practices, technological fixes, etc.). In either case, that simply puts pressure on the pathogen to adapt yet again. And on and on it would go.
But unlike the use of, say, antibiotics -- the widespread use of which just selects for an antibiotic-resistant strain of the pathogen over time -- there's nothing the pathogen can do to adapt to closed borders. Thus, closed borders are a superior solution compared to other behavioral and technological fixes.
After all, the germ can't propel itself over tens, hundreds, or thousands of miles. It needs a vector. In the extreme case where all nations have sealed shut their borders, it's literally impossible for the disease to become pandemic. Where the flow is just minimal, albeit not literally zero, we are at least selecting for a more benign pathogen. If it has a harder time reaching the next host, then it must treat its current host decently well, lest its transportation break down and leave it stranded to die out.
With all population centers large and small being connected by just hours of cheap air travel, the pathogen can treat hosts pretty badly and not get stranded. Just make sure the host doesn't drop dead in less than two hours after infection, and the pathogen can still leave them to drop dead the next day or week.
Worse, first-world elite employers of cheap labor, or their puppets in the government, are paying the cost themselves to fly in foreigners by the millions, to exploit them instead of giving domestic workers a higher standard of living. That is effectively free air travel for the foreign vector of an imported disease. Nothing spreads disease like free, fast travel.
And unlike foreign tourists, who might only spew their exotic germs for a short while in the country they're visiting, immigrants who come to live and work in their host country are going to be spreading their diseases for far longer, increasing the chance that they'll catch on. Depending on the disease, even brief tourism could do the job by itself, but immigration makes it all the more likely.
That's why in the academic article on the role of demographic interconnectedness in pandemic diseases, one key predictor of a disease being "rescued" from extinction was the share of the population who are foreigners -- not simply tourists, but foreign-born residents who immigrated. Your country could have no tourism industry to speak of, but if you've imported a decent share of your laboring class, you've got a big problem with pandemic diseases.
Of course, if you have both, like Saudi Arabia -- worldwide Muslim pilgrims to Mecca every year, plus these days nearly 40% of residents being cheap foreign labor -- then you're just asking for pandemics. And naturally, they've been hit by plague, cholera, and others in the past, so they're closing Mecca to pilgrims during the current coronavirus pandemic.
For both ecological and evolutionary reasons, nothing beats the solution of minimizing demographic flow between population clusters, or "closed borders" to be concise and dramatic. That's why we didn't need much else, if anything, during the Great Compression / New Deal period of 1920 to 1980 in order to live normal lives largely free of pandemic threats.
Conversely, that's how easy it has been to become stricken by periodic waves of pandemic disease as of 1980, in the neoliberal period -- all we had to do was throw open the borders, in the interest of elites who profit from ramping up demographic interconnectedness, particularly those who control labor-intensive economic sectors and would reap massive free profits if they could import hordes of cheap foreign labor.